Method for manufacturing semiconductor device

ABSTRACT

It is an object to provide a highly reliable semiconductor device which includes a thin film transistor having stable electric characteristics. It is another object to manufacture a highly reliable semiconductor device at lower cost with high productivity. In a method for manufacturing a semiconductor device which includes a thin film transistor where a semiconductor layer having a channel formation region, a source region, and a drain region are formed using an oxide semiconductor layer, heat treatment (heat treatment for dehydration or dehydrogenation) is performed so as to improve the purity of the oxide semiconductor layer and reduce impurities such as moisture. Moreover, the oxide semiconductor layer subjected to the heat treatment is slowly cooled under an oxygen atmosphere.

TECHNICAL FIELD

The present invention relates to a method for manufacturing asemiconductor device including an oxide semiconductor.

BACKGROUND ART

In recent years, a technique by which a thin film transistor (TFT) ismanufactured using a semiconductor thin film (having a thickness ofapproximately several nanometers to several hundred nanometers) formedover a substrate having an insulating surface has attracted attention.Thin film transistors are applied to a wide range of electronic devicessuch as ICs or electro-optical devices and urgently developedparticularly as switching elements in image display devices.

Various metal oxides are used for a variety of applications. Indiumoxide is a well-known material and is used as a transparent electrodematerial which is necessary for liquid crystal displays and the like.

Some metal oxides have semiconductor characteristics. For example, metaloxides having semiconductor characteristics include tungsten oxide, tinoxide, indium oxide, zinc oxide, and the like, and thin film transistorsin each of which a channel formation region is formed using such a metaloxide having semiconductor characteristics are already known (see PatentDocuments 1 to 4, and Non-Patent Document 1).

Further, not only single-component oxides but also multi-componentoxides are known as metal oxides. For example, InGaO₃(ZnO)_(m) (m:natural number) having a homologous series is known as a multi-componentoxide semiconductor including In, Ga, and Zn (see Non-Patent Documents 2to 4).

Further, it is confirmed that an oxide semiconductor including anIn—Ga—Zn-based oxide as described above is applicable to a channel layerof a thin film transistor (see Patent Document 5, and Non-PatentDocuments 5 and 6).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    S60-198861-   [Patent Document 2] Japanese Published Patent Application No.    H8-264794-   [Patent Document 3] Japanese Translation of PCT International    Application No. H11-505377-   [Patent Document 4] Japanese Published Patent Application No.    2000-150900-   [Patent Document 5] Japanese Published Patent Application No.    2004-103957

Non-Patent Document

-   [Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G    Muller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M.    Wolf, “A ferroelectric transparent thin-film transistor,” Appl.    Phys. Lett., 17 Jun. 1996, Vol. 68, pp. 3650-3652-   [Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J.    Solid State Chem., 1991, Vol. 93, pp. 298-315-   [Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura,    “Syntheses and Single-Crystal Data of Homologous Compounds,    In₂O₃(ZnO)_(m) (m=3, 4, and 5), InGaO₃(ZnO)₃, and Ga₂O₃(ZnO)_(m)    (m=7, 8, 9, and 16) in the In₂O₃—ZnGa₂O₄—ZnO System”, J. Solid State    Chem., 1995, Vol. 116, pp. 170-178-   [Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M.    Isobe, “Syntheses and crystal structures of new homologous    compounds, indium iron zinc oxides (InFeO₃(ZnO)_(m)) (m: natural    number) and related compounds”, KOTAI BUTSURI (SOLID STATE PHYSICS),    1993, Vol. 28, No. 5, pp. 317-327-   [Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M.    Hirano, and H. Hosono, “Thin-film transistor fabricated in    single-crystalline transparent oxide semiconductor”, SCIENCE, 2003,    Vol. 300, pp. 1269-1272-   [Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M.    Hirano, and H. Hosono, “Room-temperature fabrication of transparent    flexible thin-film transistors using amorphous oxide    semiconductors”, NATURE, 2004, Vol. 432, pp. 488-492

DISCLOSURE OF INVENTION

It is an object to manufacture and provide a highly reliablesemiconductor device including a thin film transistor having stableelectric characteristics.

In a method for manufacturing a semiconductor device which includes athin film transistor where a semiconductor layer including a channelformation region and source and drain regions are formed using an oxidesemiconductor layer, heat treatment (heat treatment for dehydration ordehydrogenation) is performed so as to improve the purity of the oxidesemiconductor layer and reduce impurities such as moisture. In addition,not only impurities such as moisture in the oxide semiconductor layerbut also those existing in a gate insulating layer are reduced, andimpurities such as moisture existing in interfaces between the oxidesemiconductor layer and films provided over and under and in contactwith the oxide semiconductor layer are reduced.

In this specification, an oxide semiconductor film which is used for asemiconductor layer including a channel formation region is referred toas a first oxide semiconductor film (a first oxide semiconductor layer),and an oxide semiconductor film which is used for source and drainregions is referred to as a second oxide semiconductor film (a secondoxide semiconductor layer).

In order to reduce impurities such as moisture, the first oxidesemiconductor film and the second oxide semiconductor film are formedand then subjected to heat treatment at 200° C. or higher, preferablyhigher than or equal to 400° C. and lower than or equal to 600° C.,under an inert gas atmosphere of nitrogen or a rare gas (such as argonor helium) or under reduced pressure, with the first oxide semiconductorfilm and the second oxide semiconductor film exposed. Thus, moistureincluded in the first oxide semiconductor film and the second oxidesemiconductor film is reduced. After the heating, the oxidesemiconductor films are slowly cooled in the range of higher than orequal to room temperature and lower than 100° C. under an oxygenatmosphere.

With the use of the first oxide semiconductor film and the second oxidesemiconductor film in which moisture is reduced by heat treatment underan inert gas atmosphere of nitrogen or argon or under reduced pressure,electric characteristics of a thin film transistor is improved and athin film transistor having both high productivity and high performanceis realized.

FIG. 29 shows results of performing thermal desorption spectroscopy(TDS) measurement using a thermal desorption spectrometer on a pluralityof samples on which heat treatment was performed under a nitrogenatmosphere under different heating-temperature conditions.

The thermal desorption spectrometer is an apparatus for detecting andidentifying, using a quadrupole mass spectrometer, a gas component whichis discharged or generated from a sample when the sample is heated andthe temperature thereof is increased in high vacuum. With the thermaldesorption spectroscopy, a gas and a molecule discharged from a surfaceor the inside of a sample can be observed. A thermal desorptionspectrometer manufactured by ESCO, Ltd. (product name: EMD-WA1000S) wasused. As for the measurement condition, the rate of temperature rise wasset at approximately 10° C./min, the SEM voltage was set to 1500 V, thedwell time was 0.2 (sec), and the number of channels to be used was 23.In addition, during the measurement, the pressure was at a degree ofvacuum of approximately 1×10⁻⁷ (Pa). Note that the ionizationcoefficient, the fragmentation coefficient, the pass-throughcoefficient, and the pumping rate of H₂O were respectively 1.0, 0.805,1.56, and 1.0.

FIG. 29 is a graph showing results of TDS measurement in terms of H₂O,where the following samples are compared: a sample (sample 1) in whichan In—Ga—Zn—O-based non-single-crystal film was formed to a thickness of50 nm over a glass substrate; a sample (sample 4) on which heattreatment was performed at 250° C. for 1 hour under a nitrogenatmosphere; a sample (sample 3) on which heat treatment was performed at350° C. for 1 hour under a nitrogen atmosphere; a sample (sample 5) onwhich heat treatment was performed at 450° C. for 1 hour under anitrogen atmosphere; and a sample (sample 6) on which heat treatment wasperformed at 350° C. for 10 hours under a nitrogen atmosphere. Theresults in FIG. 29 indicate that the higher the heating temperatureunder a nitrogen atmosphere is, the more impurities such as moisture(H₂O) which are discharged from the In—Ga—Zn—O-based non-single-crystalfilm are reduced.

In the graph of FIG. 29, a first peak showing discharge of impuritiessuch as moisture (H₂O) can be observed in the vicinity of 200° C. to250° C., and a second peak showing discharge of impurities such asmoisture (H₂O) can be observed at 300° C. or higher.

Note that even when the sample which on which heat treatment wasperformed at 450° C. under a nitrogen atmosphere is left at roomtemperature for approximately one week in air, discharge of moisture at200° C. or higher is not observed. Accordingly, it is found that theIn—Ga—Zn—O-based non-single-crystal film is stabilized by the heattreatment.

Further, TDS measurement was performed in terms of each of H, O, OH, H₂,O₂, N, N₂, and Ar in addition to H₂O. It was possible that a peak beclearly observed in terms of each of H₂O, H, O, and OH but not in termsof H₂, O₂, N, N₂, and Ar. Each sample had a structure in which anIn—Ga—Zn—O-based non-single-crystal film was formed to a thickness of 50nm over a glass substrate, and heat conditions were set as follows: at250° C. for 1 hour under a nitrogen atmosphere; at 350° C. for 1 hourunder a nitrogen atmosphere; at 350° C. for 10 hours under a nitrogenatmosphere; and at 450° C. for 1 hour under a nitrogen atmosphere. Aglass substrate alone and an In—Ga—Zn—O-based non-single-crystal film onwhich heat treatment was not performed were each subjected to themeasurement as a comparative example. FIG. 30, FIG. 31, FIG. 32, andFIG. 33 show results of TDS measurement in terms of H, O, OH, and H₂,respectively. Note that the oxygen density under a nitrogen atmospherein the above heat conditions was 20 ppm or lower.

According to the above results, it is found that moisture is mainlydischarged by heat treatment of an In—Ga—Zn—O-based non-single-crystalfilm. In other words, discharge of moisture (H₂O) from theIn—Ga—Zn—O-based non-single-crystal film is mainly caused due to heattreatment, and a product generated by decomposition of a water moleculehas an influence on values of TDS measurement in terms of H, O, and OHwhich are respectively shown in FIG. 30, FIG. 31, and FIG. 32. Note thatan In—Ga—Zn—O-based non-single-crystal film is considered to includehydrogen and OH; therefore, these are also discharged by heat treatment.

In this specification, heat treatment under an inert gas atmosphere ofnitrogen or a rare gas (such as argon or helium) or under reducedpressure is referred to as heat treatment for dehydration ordehydrogenation. In this specification, “dehydrogenation” does not referto elimination of only H₂ by heat treatment, and “dehydration ordehydrogenation” also refers to elimination of H, OH, and the like forconvenience.

After impurities (such as H₂O, H, or OH) included in the oxidesemiconductor layer are reduced by the heat treatment under an inert gasatmosphere so that the carrier density is increased, slow cooling isperformed under an oxygen atmosphere. After the slow cooling, forexample, an oxide insulating film is formed in contact with the oxidesemiconductor layer; accordingly, the carrier density of the oxidesemiconductor layer is reduced and thus reliability is increased.

The resistance of the first oxide semiconductor film and the secondoxide semiconductor film is reduced (the carrier density is increased,preferably to 1×10¹⁸/cm³ or higher) by the heat treatment under anitrogen atmosphere. Thus, the first oxide semiconductor film and thesecond oxide semiconductor film each of whose resistance is reduced canbe formed. The first oxide semiconductor film and the second oxidesemiconductor film each of whose resistance is reduced are processedthrough an etching step to form a first oxide semiconductor layer and asecond oxide semiconductor layer, and further processed through anetching step to form a semiconductor layer and source and drain regions.

After that, an oxide insulating film is formed in contact with the firstoxide semiconductor layer whose resistance is reduced, whereby in thefirst oxide semiconductor layer whose resistance is reduced, theresistance of at least a region in contact with the oxide insulatingfilm is increased (carrier density is reduced, preferably to lower than1×10¹⁸/cm³); thus, an oxide semiconductor region whose resistance isincreased can be formed. It is important to increase and reduce thecarrier density of the first oxide semiconductor film and the secondoxide semiconductor film by heating under an inert gas atmosphere (orunder reduced pressure), slow cooling under an oxygen atmosphere,formation of the oxide insulating film, and the like in a manufacturingprocess of a semiconductor device. In other words, the I-type firstoxide semiconductor film and the i-type second oxide semiconductor filmare subjected to heat treatment for dehydration or dehydrogenation to beoxygen-deficiency type, that is, n-type (such as n⁻ or n⁺-type) oxidesemiconductor films, and then the oxide insulating film is formed sothat the first oxide semiconductor layer becomes oxygen-excess type,that is, an i-type oxide semiconductor layer. Accordingly, it ispossible to manufacture and provide a semiconductor device including ahighly reliable thin film transistor having favorable electriccharacteristics.

Note that as the oxide insulating film formed to be in contact with thefirst oxide semiconductor layer whose resistance is reduced, aninorganic insulating film which blocks impurities such as moisture, ahydrogen ion, and OH⁻ is used. Specifically a silicon oxide film or asilicon nitride oxide film is used.

Further, after the oxide insulating film serving as a protective film isformed over the semiconductor layer and the source and drain regions,second heating may be performed. When the second heating is performedafter the formation of the oxide insulating film serving as a protectivefilm over the semiconductor layer and the source and drain regions,variation in electric characteristics of the thin film transistor can bereduced.

In one embodiment of the structure of the invention disclosed in thisspecification, a gate electrode layer is formed; a gate insulating layeris formed over the gate electrode layer; a first oxide semiconductorfilm is formed over the gate insulating layer; a second oxidesemiconductor film is formed over the first oxide semiconductor film;the first oxide semiconductor film and the second oxide semiconductorfilm are heated to be dedydrated or dehydrogenated and then slowlycooled under an oxygen atmosphere; the first oxide semiconductor filmand the second oxide semiconductor film which are slowly cooled underthe oxygen atmosphere are selectively etched to form a first oxidesemiconductor layer and a second oxide semiconductor layer; a conductivefilm is formed over the first oxide semiconductor layer and the secondoxide semiconductor layer; the first oxide semiconductor layer, thesecond oxide semiconductor layer, and the conductive film areselectively etched to form a semiconductor layer, a source region, adrain region, a source electrode layer, and a drain electrode layer; andan oxide insulating film which is in contact with part of thesemiconductor layer is formed over the gate insulating layer, thesemiconductor layer, the source region, the drain region, the sourceelectrode layer, and the drain electrode layer so that carrier densityis reduced.

In another embodiment of the structure of the invention disclosed inthis specification, a gate electrode layer is formed; a gate insulatinglayer is formed over the gate electrode layer; a first oxidesemiconductor film is formed over the gate insulating layer; a secondoxide semiconductor film is formed over the first oxide semiconductorfilm; the first oxide semiconductor film and the second oxidesemiconductor film are heated under an inert gas atmosphere so thatcarrier density is increased and then slowly cooled under an oxygenatmosphere; the first oxide semiconductor film and the second oxidesemiconductor film which are slowly cooled under the oxygen atmosphereare selectively etched to form a first oxide semiconductor layer and asecond oxide semiconductor layer; a conductive film is formed over thefirst oxide semiconductor layer and the second oxide semiconductorlayer; the first oxide semiconductor layer, the second oxidesemiconductor layer, and the conductive film are selectively etched toform a semiconductor layer, a source region, a drain region, a sourceelectrode layer, and a drain electrode layer; and an oxide insulatingfilm which is in contact with part of the semiconductor layer is formedover the gate insulating layer, the semiconductor layer, the sourceregion, the drain region, the source electrode layer, and the drainelectrode layer so that carrier density is reduced.

In another embodiment of the structure of the invention disclosed inthis specification, a gate electrode layer is formed; a gate insulatinglayer is formed over the gate electrode layer; a first oxidesemiconductor film is formed over the gate insulating layer; a secondoxide semiconductor film is formed over the first oxide semiconductorfilm; the first oxide semiconductor film and the second oxidesemiconductor film are heated under reduced pressure so that carrierdensity is increased and then slowly cooled under an oxygen atmosphere;the first oxide semiconductor film and the second oxide semiconductorfilm which are slowly cooled under the oxygen atmosphere are selectivelyetched to form a first oxide semiconductor layer and a second oxidesemiconductor layer; a conductive film is formed over the first oxidesemiconductor layer and the second oxide semiconductor layer; the firstoxide semiconductor layer, the second oxide semiconductor layer, and theconductive film are selectively etched to form a semiconductor layer, asource region, a drain region, a source electrode layer, and a drainelectrode layer; and an oxide insulating film which is in contact withpart of the semiconductor layer is formed over the gate insulatinglayer, the semiconductor layer, the source region, a drain region, asource electrode layer, and a drain electrode layer so that carrierdensity is reduced.

For an oxide semiconductor layer which can be used as the semiconductorlayer and the source and drain regions, an oxide material havingsemiconductor characteristics may be used. For example, thin filmsexpressed by InMO₃(ZnO)_(m) (m>0) are formed, and a thin film transistorusing the thin films as a semiconductor layer and source and drainregions is manufactured. Note that M denotes one metal element or aplurality of metal elements selected from Ga, Fe, Ni, Mn, and Co. Forexample, M denotes Ga in some cases; meanwhile, M denotes the abovemetal element such as Ni or Fe in addition to Ga (Ga and Ni or Ga andFe) in other cases. Further, the above oxide semiconductor may includeFe or Ni, another transitional metal element, or an oxide of thetransitional metal as an impurity element in addition to the metalelement included as M. In this specification, among the oxidesemiconductors whose composition formulas are expressed byInMO₃(ZnO)_(m) (m>0), an oxide semiconductor which includes Ga as M isreferred to as an In—Ga—Zn—O-based oxide semiconductor, and a thin filmof the In—Ga—Zn—O-based oxide semiconductor is also referred to as anIn—Ga—Zn—O-based non-single-crystal film.

As the oxide semiconductor which is applied to the oxide semiconductorlayer, any of the following oxide semiconductors can be applied inaddition to the above: an In—Sn—Zn—O-based oxide semiconductor; anIn—Al—Zn—O-based oxide semiconductor; a Sn—Ga—Zn—O-based oxidesemiconductor; an Al—Ga—Zn—O-based oxide semiconductor; aSn—Al—Zn—O-based oxide semiconductor; an In—Zn—O-based oxidesemiconductor; a Sn—Zn—O-based oxide semiconductor; an Al—Zn—O-basedoxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-basedoxide semiconductor; and a Zn—O-based oxide semiconductor. In addition,the above oxide semiconductor layer may include silicon oxide. Siliconoxide (SiO_(x) (x>0)), which hinders crystallization, included in theoxide semiconductor layer can suppress crystallization of the oxidesemiconductor layer in the case where heat treatment is performed afterformation of the oxide semiconductor layer in the manufacturing process.Note that the oxide semiconductor layer is preferably amorphous but maybe partly crystallized.

The oxide semiconductor layer is preferably an oxide semiconductorcontaining In, more preferably an oxide semiconductor containing In andGa. In order to obtain an I-type (intrinsic) oxide semiconductor,dehydration or dehydrogenation is effective.

In addition, the oxide semiconductor layer used as the source region andthe drain region (also referred to as n⁺ layers or buffer layers) of thethin film transistor preferably has higher conductivity (electricalconductivity) than the oxide semiconductor layer used as a channelformation region.

Since a thin film transistor is easily broken due to static electricityor the like, a protective circuit for protecting a driver circuit ispreferably provided over the same substrate for a gate line or a sourceline. The protective circuit is preferably formed using a non-linearelement including an oxide semiconductor.

The gate insulating layer, the first oxide semiconductor film, and thesecond oxide semiconductor film may be successively treated (alsoreferred to as successive treatment, an insitu process, or successivefilm formation) without exposure to air. By the successive treatmentwithout exposure to air, each interface between the stacked layers, thatis, interfaces of the gate insulating layer, the first oxidesemiconductor film, and the second oxide semiconductor film can beformed without being contaminated by an atmospheric component or animpurity element floating in air, such as water or hydrocarbon.Accordingly, variation in characteristics of the thin film transistorcan be reduced.

Note that the term “successive treatment” in this specification meansthat during a process from a first treatment step using a PCVD method ora sputtering method to a second treatment step using a PCVD method or asputtering method, an atmosphere in which a substrate to be processed isdisposed is not contaminated by a contaminant atmosphere such as air,and is constantly controlled to be vacuum or an inert gas atmosphere (anitrogen atmosphere or a rare gas atmosphere). By the successivetreatment, treatment such as film formation can be performed whilemoisture or the like is prevented from attaching again to the cleanedsubstrate to be processed.

Performing the process from the first treatment step to the secondtreatment step in the same chamber is within the scope of the successivetreatment in this specification.

In addition, the following is also within the scope of the successivetreatment in this specification: in the case of performing the processfrom the first treatment step to the second treatment step in differentchambers, the substrate is transferred after the first treatment step toanother chamber without exposure to air and is subjected to the secondtreatment.

Note that between the first treatment step and the second treatmentstep, a substrate transfer step, an alignment step, a slow-cooling step,a step of heating or cooling the substrate to a temperature which isnecessary for the second treatment step, or the like may be provided.Such a process is also within the scope of the successive treatment inthis specification.

A step in which liquid is used, such as a cleaning step, wet etching, orresist formation, may be provided between the first treatment step andthe second treatment step. This case is not within the scope of thesuccessive treatment in this specification.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify theinvention.

Moreover, as a display device including a driver circuit, alight-emitting display device including a light-emitting element and adisplay device including an electrophoretic display element, which isalso referred to as electronic paper, are given in addition to a liquidcrystal display device.

In the light-emitting display device including a light-emitting element,a plurality of thin film transistors are included in a pixel portion,and in the pixel portion, there is a region where a gate electrode of athin film transistor is connected to a source wiring or a drain wiringof another thin film transistor. In addition, in a driver circuit of thelight-emitting display device including a light-emitting element, thereis a region where a gate electrode of a thin film transistor isconnected to a source wiring or a drain wiring of the thin filmtransistor.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectro-optical device, a semiconductor circuit, and an electronicappliance are all semiconductor devices.

A thin film transistor having stable electric characteristics can bemanufactured and provided. Accordingly, a semiconductor device includingthe highly reliable thin film transistor having favorable electriccharacteristics can be provided.

BRIEF DESCRIPTION OF DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication with thecolor drawings will be provided by the Office upon request and paymentof the necessary fee.

In the accompanying drawings:

FIGS. 1A to 1C illustrate a method for manufacturing a semiconductordevice;

FIGS. 2A and 2B illustrate the method for manufacturing a semiconductordevice;

FIGS. 3A and 3B illustrate a semiconductor device;

FIGS. 4A to 4C illustrate a method for manufacturing a semiconductordevice;

FIGS. 5A to 5C illustrate the method for manufacturing a semiconductordevice;

FIGS. 6A and 6B illustrate the method for manufacturing a semiconductordevice;

FIG. 7 illustrates a semiconductor device;

FIGS. 8A1, 8A2, 8B1, and 8B2 illustrate semiconductor devices;

FIG. 9 illustrates a semiconductor device;

FIGS. 10A1, 10A2, and 10B illustrate a semiconductor device;

FIGS. 11A and 11B illustrate a semiconductor device;

FIG. 12 illustrates a pixel equivalent circuit of a semiconductordevice;

FIGS. 13A to 13C illustrate semiconductor devices;

FIGS. 14A and 14B are block diagrams of semiconductor devices;

FIG. 15 illustrates a configuration of a signal line driver circuit;

FIG. 16 is a timing chart illustrating operation of a signal line drivercircuit;

FIG. 17 is a timing chart illustrating operation of a signal line drivercircuit;

FIG. 18 illustrates a configuration of a shift register;

FIG. 19 illustrates a connection structure of the flip-flop illustratedin FIG. 18;

FIG. 20 illustrates a semiconductor device;

FIG. 21 shows simulation results of oxygen density of an oxidesemiconductor layer;

FIG. 22 is an external view illustrating an example of an electronicbook reader;

FIGS. 23A and 23B are external views illustrating examples of atelevision set and a digital photo frame, respectively;

FIGS. 24A and 24B are external views illustrating examples of amusementmachines;

FIGS. 25A and 25B are external views respectively illustrating anexample of a portable computer and an example of a cellular phone;

FIG. 26 illustrates a semiconductor device;

FIG. 27 illustrates a semiconductor device;

FIG. 28 is a cross-sectional view illustrating an electric furnace;

FIG. 29 is a graph showing results of TDS measurement;

FIG. 30 is a graph showing results of TDS measurement in terms of H;

FIG. 31 is a graph showing results of TDS measurement in terms of 0;

FIG. 32 is a graph showing results of TDS measurement in terms of OH;

FIG. 33 is a graph showing results of TDS measurement in terms of H₂;and

FIG. 34 illustrates a structure of an oxide semiconductor layer used forsimulation.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and an example will be described in detail with reference todrawings. However, the present invention is not limited to the followingdescription, and it is easily understood by those skilled in the artthat various changes and modifications can be made in modes and detailswithout departing from the spirit and scope of the present invention.Therefore, the present invention should not be interpreted as beinglimited to the description of the embodiments and example below. In thestructures described below, the same portions or portions having similarfunctions are denoted by the same reference numerals in differentdrawings, and repetitive description thereof is omitted.

Embodiment 1

A semiconductor device and a method for manufacturing the semiconductordevice will be described with reference to FIGS. 1A to 1C, FIGS. 2A and2B, and FIGS. 3A and 3B.

FIG. 3A is a plan view of a thin film transistor 470 included in asemiconductor device, and FIG. 3B is a cross-sectional view taken alongline C1-C2 of FIG. 3A. The thin film transistor 470 is an invertedstaggered thin film transistor and includes, over a substrate 400 whichis a substrate having an insulating surface, a gate electrode layer 401,a gate insulating layer 402, a semiconductor layer 403, source and drainregions 404 a and 404 b, and source and drain electrode layers 405 a and405 b. In addition, an oxide insulating film 407 is provided to coverthe thin film transistor 470 and is in contact with the semiconductorlayer 403.

Heat treatment (heat treatment for dehydration or dehydrogenation) whichreduces impurities such as moisture is performed at least afterformation of a first oxide semiconductor film and a second oxidesemiconductor film which become the semiconductor layer 403 and thesource and drain regions 404 a and 404 b, so that the resistance of thefirst oxide semiconductor film and the second oxide semiconductor filmis reduced (the carrier density is increased, preferably to 1×10¹⁸/cm³or higher). Then, the oxide insulating film 407 is formed in contactwith a first oxide semiconductor layer, whereby the resistance thereofis increased (the carrier density is reduced, preferably to lower than1×10¹⁸/cm³, more preferably to lower than or equal to 1×10¹⁴/cm³). Thus,the first oxide semiconductor layer whose resistance is increased can beused as a channel formation region.

Further, after impurities such as moisture (H₂O) are eliminated by theheat treatment for dehydration or dehydrogenation, slow cooling ispreferably performed under an oxygen atmosphere. After the heattreatment for dehydration or dehydrogenation and the slow cooling underthe oxygen atmosphere, the carrier density of the first oxidesemiconductor layer is reduced by formation of the oxide insulating filmto be in contact with the first oxide semiconductor layer, which resultsin that reliability of the thin film transistor 470 is increased.

Impurities such as moisture are reduced not only in the semiconductorlayer 403 and the source and drain regions 404 a and 404 b but also inthe gate insulating layer 402 and interfaces between the semiconductorlayer 403 which is an oxide semiconductor layer and films that areprovided over and under and in contact with the semiconductor layer 403,specifically, an interface between the gate insulating layer 402 and thesemiconductor layer 403 and an interface between the oxide insulatingfilm 407 and the semiconductor layer 403.

Note that the source and drain electrode layers 405 a and 405 b that arein contact with the semiconductor layer 403 which is an oxidesemiconductor layer and the source and drain regions 404 a and 404 b areformed using one or more materials selected from titanium, aluminum,manganese, magnesium, zirconium, beryllium, and thorium. Further, astack of alloy films including any combination of the above elements maybe used.

For the semiconductor layer 403 including a channel formation region andthe source and drain regions 404 a and 404 b, an oxide material havingsemiconductor characteristics may be used. For example, an oxidesemiconductor having a structure expressed by InMO₃(ZnO)_(m) (m>0) canbe used, and in particular, an In—Ga—Zn—O-based oxide semiconductor ispreferably used. Note that M denotes one metal element or a plurality ofmetal elements selected from gallium (Ga), iron (Fe), nickel (Ni),manganese (Mn), and cobalt (Co). For example, M denotes Ga in somecases; meanwhile, M denotes the above metal element such as Ni or Fe inaddition to Ga (Ga and Ni or Ga and Fe) in other cases. Further, theabove oxide semiconductor may include Fe or Ni, another transitionalmetal element, or an oxide of the transitional metal as an impurityelement in addition to the metal element included as M. In thisspecification, among the oxide semiconductors whose composition formulasare expressed by InMO₃(ZnO)_(m) (m>0), an oxide semiconductor whichincludes at least Ga as M is referred to as an In—Ga—Zn—O-based oxidesemiconductor, and a thin film of the In—Ga—Zn—O-based oxidesemiconductor is also referred to as an In—Ga—Zn—O-basednon-single-crystal film.

As the oxide semiconductor which is applied to the oxide semiconductorlayer, any of the following oxide semiconductors can be applied inaddition to the above: an In—Sn—Zn—O-based oxide semiconductor; anIn—Al—Zn—O-based oxide semiconductor; a Sn—Ga—Zn—O-based oxidesemiconductor; an Al—Ga—Zn—O-based oxide semiconductor; aSn—Al—Zn—O-based oxide semiconductor; an In—Zn—O-based oxidesemiconductor; a Sn—Zn—O-based oxide semiconductor; an Al—Zn—O-basedoxide semiconductor; an In—O-based oxide semiconductor; a Sn—O-basedoxide semiconductor; and a Zn—O-based oxide semiconductor. In addition,the above oxide semiconductor may include silicon oxide.

The source region is provided between the semiconductor layer (alsoreferred to as the first oxide semiconductor layer) and the sourceelectrode layer, and the drain region is provided between thesemiconductor layer and the drain electrode layer. As the source anddrain regions, an oxide semiconductor layer having n-type conductivity(also referred to as the second oxide semiconductor layer) can be used.

In addition, it is preferable that the second oxide semiconductor layerused as the source and drain regions 404 a and 404 b of the thin filmtransistor be thinner and have higher conductivity (electricalconductivity) than the first oxide semiconductor layer used as a channelformation region.

Further, the first oxide semiconductor layer used as the channelformation region has an amorphous structure and the second oxidesemiconductor layer used as the source and drain regions includes acrystal grain (nanocrystal) in the amorphous structure in some cases.The crystal grain (nanocrystal) in the second oxide semiconductor layerused as the source and drain regions has a diameter of 1 nm to 10 nm,typically 2 nm to 4 nm, approximately.

In this embodiment, In—Ga—Zn—O-based non-single-crystal films are usedas the semiconductor layer 403 including the channel formation regionand the source and drain regions (also referred to as n⁺ layers orbuffer layers) 404 a and 404 b.

FIGS. 1A to 1C and FIGS. 2A and 2B are cross-sectional viewsillustrating a manufacturing process of the thin film transistor 470.

The gate electrode layer 401 is provided over the substrate 400 which isa substrate having an insulating surface. An insulating film serving asa base film may be provided between the substrate 400 and the gateelectrode layer 401. The base film has a function of preventingdiffusion of an impurity element from the substrate 400, and can beformed to have a single-layer or stacked-layer structure using one ormore films selected from a silicon nitride film, a silicon oxide film, asilicon nitride oxide film, and a silicon oxynitride film. The gateelectrode layer 401 can be formed to have a single-layer orstacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material containing any of these materials as itsmain component.

For example, as a stacked-layer structure of two layers of the gateelectrode layer 401, the following structures are preferable: atwo-layer structure of an aluminum layer and a molybdenum layer stackedthereover; a two-layer structure of a copper layer and a molybdenumlayer stacked thereover; a two-layer structure of a copper layer and atitanium nitride layer or a tantalum nitride layer stacked thereover;and a two-layer structure of a titanium nitride layer and a molybdenumlayer. As a stacked-layer structure of three layers, a stack of atungsten layer or a tungsten nitride layer, a layer of an alloy ofaluminum and silicon or a layer of an alloy of aluminum and titanium,and a titanium nitride layer or a titanium layer is preferable.

The gate insulating layer 402 is formed over the gate electrode layer401.

The gate insulating layer 402 can be formed using a single layer orstacked layers of any of a silicon oxide layer, a silicon nitride layer,a silicon oxynitride layer, and a silicon nitride oxide layer with aplasma CVD method, a sputtering method, or the like. For example, asilicon oxynitride layer may be formed with a plasma CVD method usingSiH₄, oxygen, and nitrogen as a film-formation gas.

A first oxide semiconductor film 430 and a second oxide semiconductorfilm 433 are formed to be stacked over the gate insulating layer 402(see FIG. 1A). The first oxide semiconductor film 430 serves as asemiconductor layer which functions as a channel formation region, andthe second oxide semiconductor film 433 serves as source and drainregions.

Note that before the oxide semiconductor films are formed with asputtering method, reverse sputtering in which an argon gas isintroduced and plasma is generated is preferably performed to removedust attached to a surface of the gate insulating layer 402. The reversesputtering refers to a method in which, without application of voltageto a target side, an RF power source is used for application of voltageto a substrate side under an argon atmosphere to generate plasma in thevicinity of the substrate so that a surface is modified. Note thatinstead of an argon atmosphere, a nitrogen atmosphere, a heliumatmosphere, or the like may be used. Alternatively, an argon atmosphereto which oxygen, N₂O, or the like is added may be used. Furtheralternatively, an argon atmosphere to which Cl₂, CF₄, or the like isadded may be used.

As the oxide semiconductor films, In—Ga—Zn—O-based non-single-crystalfilms are used. The oxide semiconductor films are formed with asputtering method using an In—Ga—Zn—O-based oxide semiconductor target.The oxide semiconductor films can be formed with a sputtering methodunder a rare gas (typically argon) atmosphere, an oxygen atmosphere, oran atmosphere of a rare gas (typically argon) and oxygen.

The gate insulating layer 402, the first oxide semiconductor film 430,and the second oxide semiconductor film 433 may be successively formedwithout exposure to air. By successive film formation without exposureto air, each interface of the stacked layers can be formed without beingcontaminated by an atmospheric component or an impurity element floatingin air, such as water or hydrocarbon. Therefore, variation incharacteristics of the thin film transistor can be reduced.

The first oxide semiconductor film 430 and the second oxidesemiconductor film 433 are subjected to heat treatment under anatmosphere of an inert gas (such as nitrogen, helium, neon, or argon) orunder reduced pressure, and then slowly cooled under an oxygenatmosphere (see FIG. 1B). When the heat treatment under the aboveatmosphere is performed on the first oxide semiconductor film 430 andthe second oxide semiconductor film 433, impurities such as moisture orhydrogen which are included in the first oxide semiconductor film 430and the second oxide semiconductor film 433 can be removed.

Note that it is preferable that impurities such as moisture or hydrogenbe not included in nitrogen or a rare gas such as helium, neon, or argonin the heat treatment. In addition, nitrogen or a rare gas such ashelium, neon, or argon which is introduced into a heat treatmentapparatus preferably has a purity of 6N (99.9999%) or higher, morepreferably 7N (99.99999%) or higher (that is, the density of impuritiesis 1 ppm or lower, preferably 0.1 ppm or lower).

Further, in the heat treatment, a heating method using an electricfurnace or an instantaneous heating method such as a gas rapid thermalanneal (GRTA) method using a heated gas or a lamp rapid thermal anneal(LRTA) method can be used.

Here, as one mode of the heat treatment for the first oxidesemiconductor film 430 and the second oxide semiconductor film 433, aheating method using an electric furnace 601 is described with referenceto FIG. 28.

FIG. 28 is a schematic view of the electric furnace 601. A heater 603 isprovided outside a chamber 602 and used for heating the chamber 602. Asusceptor 605 on which a substrate 604 is set is provided in the chamber602, and the substrate 604 is carried into or out of the chamber 602.Further, the chamber 602 is provided with a gas supply unit 606 and anevacuation unit 607. A gas is introduced into the chamber 602 by the gassupply unit 606. The evacuation unit 607 evacuates the chamber 602 orreduces the pressure in the chamber 602. Note that the temperature risecharacteristics of the electric furnace 601 is preferably set at higherthan or equal to 0.1° C./min and lower than or equal to 20° C./min. Inaddition, the temperature drop characteristics of the electric furnace601 is preferably set at higher than or equal to 0.1° C./min and lowerthan or equal to 15° C./min.

The gas supply unit 606 includes a gas supply source 611 a, a gas supplysource 611 b, a pressure adjusting valve 612 a, a pressure adjustingvalve 612 b, a refiner 613 a, a refiner 613 b, a mass flow controller614 a, a mass flow controller 614 b, a stop valve 615 a, and a stopvalve 615 b. In this embodiment, the refiner 613 a and the refiner 613 bare preferably provided between the gas supply source 611 a and the gassupply source 611 b, and the chamber 602. With the refiner 613 a and therefiner 613 b, impurities such as moisture or hydrogen in a gas which isintroduced into the chamber 602 from the gas supply source 611 a and thegas supply source 611 b can be removed by the refiner 613 a and therefiner 613 b, so that entry of impurities such as moisture or hydrogeninto the chamber 602 can be suppressed.

In this embodiment, nitrogen or a rare gas is introduced into thechamber 602 from the gas supply source 611 a and the gas supply source611 b so that the atmosphere in the chamber is a nitrogen or rare gasatmosphere, and the first oxide semiconductor film 430 and the secondoxide semiconductor film 433 which are formed over the substrate 604 areheated in the chamber 602 heated to higher than or equal to 200° C. andlower than or equal to 600° C., preferably higher than or equal to 400°C. and lower than or equal to 450° C. In this manner, dehydration ordehydrogenation of the first oxide semiconductor film 430 and the secondoxide semiconductor film 433 can be performed.

Alternatively, dehydration or dehydrogenation of the first oxidesemiconductor film 430 and the second oxide semiconductor film 433 canbe performed in such a manner that, with the pressure reduced by theevacuation unit, the first oxide semiconductor film 430 and the secondoxide semiconductor film 433 which are formed over the substrate 604 areheated in the chamber 602 heated to higher than or equal to 200° C. andlower than or equal to 600° C., preferably higher than or equal to 400°C. and lower than or equal to 450° C.

Next, introduction of nitrogen or a rare gas from the gas supply source611 a into the chamber 602 is stopped, and further, the heater is turnedoff. Then, oxygen is introduced into the chamber 602 from the gas supplysource 611 b, and the chamber 602 of the heating apparatus is slowlycooled. That is, the atmosphere in the chamber 602 is changed to anoxygen atmosphere, and the substrate 604 is slowly cooled. Here, it ispreferable that impurities such as moisture or hydrogen be not includedin oxygen which is introduced into the chamber 602 from the gas supplysource 611 b. In addition, oxygen which is introduced into the chamber602 from the gas supply source 611 b preferably has a purity of 6N(99.9999%) or higher, more preferably 7N (99.99999%) or higher (that is,the density of impurities in the oxygen is 1 ppm or lower, preferably0.1 ppm or lower). By the heat treatment under an inert gas atmosphereor under reduced pressure, the resistance of the oxide semiconductorfilms is reduced (the carrier density is increased, preferably to1×10¹⁸/cm³ or higher). Thus, a first oxide semiconductor film 434 and asecond oxide semiconductor film 435 each of whose resistance is reducedcan be formed.

As a result, reliability of the thin film transistor to be completedlater can be improved.

Note that when the heat treatment is performed under reduced pressure,cooling may be performed by introducing oxygen into the chamber 602after the heat treatment so that the pressure returns to atmosphericpressure.

In addition, at the same time as introduction of oxygen into the chamber602 from the gas supply source 611 b, one of or both nitrogen and a raregas such as helium, neon, or argon may be introduced into the chamber602.

Alternatively, after the substrate 604 in the chamber 602 of the heatingapparatus is cooled to 300° C., the substrate 604 may be transferredinto an atmosphere of room temperature. This results in that coolingtime for the substrate 604 can be shortened.

When the heating apparatus has multiple chambers, the heat treatment andthe cooling treatment can be performed in different chambers. Typically,in a first chamber which is filled with nitrogen or a rare gas andheated to higher than or equal to 200° C. and lower than or equal to600° C., preferably higher than or equal to 400° C. and lower than orequal to 450° C., the oxide semiconductor films over the substrate areheated. Next, the substrate on which the above heat treatment isperformed is transferred, through a transfer chamber into which nitrogenor an inert gas is introduced, to a second chamber which is filled withoxygen and whose temperature is lower than or equal to 100° C.,preferably room temperature, and is subjected to cooling treatment. Bythe above process, throughput can be improved.

After the heat treatment under an inert gas atmosphere or under reducedpressure, slow cooling to higher than or equal to room temperature andlower than 100° C. is performed under an oxygen atmosphere, thesubstrate provided with the first oxide semiconductor film 434 and thesecond oxide semiconductor film 435 is taken out of the heatingapparatus, and a photolithography step is performed.

The first oxide semiconductor film 434 and the second oxidesemiconductor film 435 after the heat treatment under an inert gasatmosphere or under reduced pressure are preferably in an amorphousstate, but may be partly crystallized.

The first oxide semiconductor film 434 and the second oxidesemiconductor film 435 are processed into a first oxide semiconductorlayer 431 and a second oxide semiconductor layer 436 which areisland-like oxide semiconductor layers through the photolithography step(see FIG. 1C).

A conductive film is formed over the gate insulating layer 402, thefirst oxide semiconductor layer 431, and the second oxide semiconductorlayer 436.

As a material for the conductive film, an element selected from Al, Cr,Ta, Ti, Mo, and W; an alloy containing any of the above elements as itscomponent; an alloy film containing a combination of any of the aboveelements; and the like can be given.

In the case where heat treatment is performed after the formation of theconductive film, it is preferable that the conductive film have heatresistance enough to withstand the heat treatment. Since Al alone hasdisadvantages such as low heat resistance and a tendency to be corroded,aluminum is used in combination with a heat-resistant conductivematerial. As the heat-resistant conductive material which is used incombination with Al, any of the following materials may be used: anelement selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc); analloy containing any of the above elements as a component; an alloy filmcontaining a combination of any of the above elements; and a nitridecontaining any of the above elements as a component.

The first oxide semiconductor layer 431, the second oxide semiconductorlayer 436, and the conductive film are etched through an etching step toform a first oxide semiconductor layer 432, the source and drain regions404 a and 404 b, and the source and drain electrode layers 405 a and 405b (see FIG. 2A). Note that only part of the first oxide semiconductorlayer 431 is etched, so that the first oxide semiconductor layer 432 hasa depression (a recessed portion).

A silicon oxide film is formed as the oxide insulating film 407 with asputtering method so as to be in contact with the first oxidesemiconductor layer 432. The oxide insulating film 407 which is formedto be in contact with the oxide semiconductor layer whose resistance isreduced does not contain impurities such as moisture, a hydrogen ion,and OH⁻ and is formed using an inorganic insulating film which preventsthe impurities from entering from the outside. Specifically, a siliconoxide film or a silicon nitride oxide film is used.

In this embodiment, a silicon oxide film is formed to a thickness of 300nm as the oxide insulating film 407. The substrate temperature in thefilm formation may be higher than or equal to room temperature and lowerthan or equal to 300° C., and is set at 100° C. in this embodiment.Formation of the silicon oxide film with a sputtering method can beperformed under a rare gas (typically argon) atmosphere, an oxygenatmosphere, or an atmosphere of a rare gas (typically argon) and oxygen.Further, either a silicon oxide target or a silicon target may be usedas a target. For example, the silicon oxide film can be formed with asputtering method using a silicon target under an atmosphere of oxygenand nitrogen.

When the oxide insulating film 407 is formed with a sputtering method, aPCVD method, or the like so as to be in contact with the first oxidesemiconductor layer 432 whose resistance is reduced, in the first oxidesemiconductor layer 432 whose resistance is reduced, the resistance ofat least a region in contact with the oxide insulating film 407 isincreased (the carrier density is reduced, preferably to lower than1×10¹⁸/cm³, more preferably to lower than or equal to 1×10¹⁴/cm³); thus,an oxide semiconductor region whose resistance is increased can beformed. It is important to increase and reduce the carrier density ofthe oxide semiconductor layer by heating under an inert gas atmosphere(or under reduced pressure), slow cooling under an oxygen atmosphere,formation of the oxide insulating film, and the like in a manufacturingprocess of the semiconductor device. The first oxide semiconductor layer432 serves as the semiconductor layer 403 including the oxidesemiconductor region whose resistance is increased. In this manner, thethin film transistor 470 can be manufactured (see FIG. 2B).

Impurities (such as H₂O, H, or OH) included in the first oxidesemiconductor film and the second oxide semiconductor film are reducedby the above heat treatment for dehydration or dehydrogenation so thatthe carrier density is increased, and then slow cooling is performedunder an oxygen atmosphere. After the slow cooling, the first oxidesemiconductor film is processed into an island-like first oxidesemiconductor layer, and an oxide insulating film is formed in contactwith the first oxide semiconductor layer so that the carrier density ofthe first oxide semiconductor layer is reduced. When the first oxidesemiconductor layer whose carrier density is reduced is used as asemiconductor layer, reliability of the thin film transistor 470 can beimproved.

Further, after the oxide insulating film 407 is formed, the thin filmtransistor 470 may be subjected to heat treatment (preferably at higherthan or equal to 150° C. and lower than 350° C.) under a nitrogenatmosphere or an air atmosphere (in air). For example, heat treatment isperformed at 250° C. for 1 hour under a nitrogen atmosphere. By the heattreatment, the semiconductor layer 403 is heated while being in contactwith the oxide insulating film 407. Accordingly, variation in electriccharacteristics of the thin film transistor 470 can be reduced. There isno particular limitation on this heat treatment (preferably at higherthan or equal to 150° C. and lower than 350° C.) as long as it isperformed after the formation of the oxide insulating film 407. When theheat treatment serves also as another step such as heat treatment information of a resin film or heat treatment for reducing the resistanceof a transparent conductive film, the heat treatment can be performedwithout increase in the number of steps.

Embodiment 2

A semiconductor device and a method for manufacturing the semiconductordevice will be described with reference to FIG. 26. The same portions asEmbodiment 1 or portions having functions similar to those of Embodiment1 can be formed in a manner similar to that of Embodiment 1, andrepetitive description thereof is omitted.

A thin film transistor 471 illustrated in FIG. 26 is an example in whicha conductive layer 408 is provided so as to overlap with the gateelectrode layer 401 and a channel region of the semiconductor layer 403with an insulating film interposed between the conductive layer 408 andthe semiconductor layer 403.

FIG. 26 is a cross-sectional view of the thin film transistor 471included in a semiconductor device. The thin film transistor 471 is abottom-gate thin film transistor in which, over the substrate 400 whichis a substrate having an insulating surface, the gate electrode layer401, the gate insulating layer 402, the semiconductor layer 403, thesource and drain regions 404 a and 404 b, the source and drain electrodelayers 405 a and 405 b, and the conductive layer 408 are provided. Theconductive layer 408 is provided over the oxide insulating film 407 soas to overlap with the gate electrode layer 401.

The conductive layer 408 can be formed using a material and a methodsimilar to those of the gate electrode layer 401 and the source anddrain electrode layers 405 a and 405 b. In the case where a pixelelectrode layer is provided, the conductive layer 408 can be formedusing a material and a method similar to those of the pixel electrodelayer. In this embodiment, a stack of a titanium film, an aluminum film,and a titanium film is used for the conductive layer 408.

The conductive layer 408 may have potential which is the same as ordifferent from that of the gate electrode layer 401, and can function asa second gate electrode layer. Further, the conductive layer 408 may bein a floating state.

When the conductive layer 408 is provided in a position overlapping withthe semiconductor layer 403, in a bias-temperature stress test(hereinafter referred to as a BT test) for examining reliability of athin film transistor, the amount of fluctuation in threshold voltage ofthe thin film transistor 471 before and after the BT test can bereduced. Especially in a −BT test in which a voltage of −20 V is appliedto a gate after the substrate temperature is raised to 150° C.,fluctuation in the threshold voltage can be suppressed.

This embodiment can be implemented in appropriate combination withEmbodiment 1.

Embodiment 3

A semiconductor device and a method for manufacturing the semiconductordevice will be described with reference to FIG. 27. The same portions asEmbodiment 1 or portions having functions similar to those of Embodiment1 can be formed in a manner similar to that of Embodiment 1, andrepetitive description thereof is omitted.

A thin film transistor 472 illustrated in FIG. 27 is an example in whicha conductive layer 409 is provided so as to overlap with the gateelectrode layer 401 and a channel region of the semiconductor layer 403with the oxide insulating film 407 and an insulating layer 410interposed between the conductive layer 409 and the semiconductor layer403.

FIG. 27 is a cross-sectional view of the thin film transistor 472included in a semiconductor device. The thin film transistor 472 is abottom-gate thin film transistor in which, over the substrate 400 whichis a substrate having an insulating surface, the gate electrode layer401, the gate insulating layer 402, the semiconductor layer 403, thesource and drain regions 404 a and 404 b, the source and drain electrodelayers 405 a and 405 b, and the conductive layer 409 are provided. Theconductive layer 409 is provided over the oxide insulating film 407 andthe insulating layer 410 so as to overlap with the gate electrode layer401.

In this embodiment, the insulating layer 410 which functions as aplanarization film is stacked over the oxide insulating film 407, and anopening which reaches the source or drain electrode layer 405 b isformed in the oxide insulating film 407 and the insulating layer 410. Aconductive film is formed in the opening which is formed in the oxideinsulating film 407 and the insulating layer 410 and etched into adesired shape, so that the conductive layer 409 and a pixel electrodelayer 411 are formed. In this manner, the conductive layer 409 can beformed in the step of forming the pixel electrode layer 411, using asimilar material and method. In this embodiment, indium oxide-tin oxidealloy including silicon oxide (an In—Sn—O-based oxide including siliconoxide) is used for the pixel electrode layer 411 and the conductivelayer 409.

Alternatively, the conductive layer 409 may be formed using a materialand a method similar to those of the gate electrode layer 401 and thesource and drain electrode layers 405 a and 405 b.

The conductive layer 409 may have potential which is the same as ordifferent from that of the gate electrode layer 401, and can function asa second gate electrode layer. Further, the conductive layer 409 may bein a floating state.

When the conductive layer 409 is provided in a position overlapping withthe semiconductor layer 403, in a bias-temperature stress test(hereinafter referred to as a BT test) for examining reliability of athin film transistor, the amount of fluctuation in threshold voltage ofthe thin film transistor 472 before and after the BT test can bereduced.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 4

A manufacturing process of a semiconductor device including a thin filmtransistor will be described with reference to FIGS. 4A to 4C, FIGS. 5Ato 5C, FIGS. 6A and 6B, FIG. 7, and FIGS. 8A1, 8A2, 8B1, and 8B2.

In FIG. 4A, as a substrate 100 having a light-transmitting property, aglass substrate of barium borosilicate glass, aluminoborosilicate glass,or the like can be used.

Next, a conductive layer is formed entirely over a surface of thesubstrate 100, and then a first photolithography process is performed. Aresist mask is formed and unnecessary portions are removed by etching,so that wirings and an electrode (a gate wiring including a gateelectrode layer 101, a capacitor wiring 108, and a first terminal 121)are formed. At this time, the etching is performed so that at least endportions of the gate electrode layer 101 are tapered.

Each of the gate wiring including the gate electrode layer 101, thecapacitor wiring 108, and the first terminal 121 in a terminal portionis preferably formed using a heat-resistant conductive material such asan element selected from titanium (Ti), tantalum (Ta), tungsten (W),molybdenum (Mo), chromium (Cr), neodymium (Nd), and scandium (Sc); analloy containing any of these elements as its component; an alloy filmcontaining a combination of any of the above elements; or a nitridecontaining any of the above elements as its component. In the case wherea low-resistant conductive material such as aluminum (Al) or copper (Cu)is used, the low-resistant conductive material is used in combinationwith the above heat-resistant conductive material because Al alone or Cualone has disadvantages such as low heat resistance and a tendency to becorroded.

Next, a gate insulating layer 102 is formed entirely over a surface ofthe gate electrode layer 101 (see FIG. 4A). The gate insulating layer102 is formed to a thickness of 50 nm to 250 nm with a sputteringmethod, a PCVD method, or the like.

For example, as the gate insulating layer 102, a silicon oxide film isformed to a thickness of 100 nm with a sputtering method. Needless tosay, the gate insulating layer 102 is not limited to such a siliconoxide film and may be formed to have a single-layer structure or astacked-layer structure using another insulating film such as a siliconoxynitride film, a silicon nitride film, an aluminum oxide film, or atantalum oxide film.

Next, a first oxide semiconductor film 131 (a first In—Ga—Zn—O-basednon-single-crystal film) is formed over the gate insulating layer 102.The first oxide semiconductor film 131 is formed without exposure to airafter plasma treatment, which is advantageous in that dust or moistureis not attached to an interface between the gate insulating layer andthe semiconductor film. Here, the first oxide semiconductor film 131 isformed under an argon or oxygen atmosphere using an oxide semiconductortarget having a diameter of 8 inches and including In, Ga, and Zn(In₂O₃:Ga₂O₃:ZnO=1:1:1), with the distance between the substrate and thetarget set to 170 mm, under a pressure of 0.4 Pa, and with a directcurrent (DC) power source of 0.5 kW. Note that a pulse direct current(DC) power source is preferable because dust can be reduced and the filmthickness can be uniform. The thickness of the first oxide semiconductorfilm 131 is set to 5 nm to 200 nm As the first oxide semiconductor film131, an In—Ga—Zn—O-based non-single-crystal film is formed to athickness of 50 nm with a sputtering method using an In—Ga—Zn—O-basedoxide semiconductor target.

Next, a second oxide semiconductor film 136 (a second In—Ga—Zn—O-basednon-single-crystal film) is formed with a sputtering method withoutexposure to air (see FIG. 4B). Here, sputtering is performed using atarget of In₂O₃:Ga₂O₃:ZnO=1:1:1 under film formation conditions wherethe pressure is 0.4 Pa, the power is 500 W, the film formationtemperature is room temperature, and an argon gas is introduced at aflow rate of 40 sccm. Although the target of In₂O₃:Ga₂O₃:ZnO=1:1:1 isused, an In—Ga—Zn—O-based non-single-crystal film including a crystalgrain which has a size of 1 nm to 10 nm just after the film formation isobtained in some cases. Note that it can be said that the presence orabsence of crystal grains or the density of crystal grains can beadjusted and the diameter size can be adjusted within the range of 1 nmto 10 nm by appropriate adjustment of the film formation conditions ofreactive sputtering, such as the composition ratio in the target, thefilm formation pressure (0.1 Pa to 2.0 Pa), the power (250 W to 3000 W:8 inches φ), and the temperature (room temperature to 100° C.). Thesecond In—Ga—Zn—O-based non-single-crystal film has a thickness of 5 nmto 20 nm. Needless to say, the size of a crystal grain included in thefilm does not exceed the film thickness. Here, the thickness of thesecond In—Ga—Zn—O-based non-single-crystal film is 5 nm.

The first In—Ga—Zn—O-based non-single-crystal film is formed under filmformation conditions different from those for the secondIn—Ga—Zn—O-based non-single-crystal film. For example, the firstIn—Ga—Zn—O-based non-single-crystal film is formed under conditionswhere the ratio of an oxygen gas flow rate to an argon gas flow rate ishigher than the ratio of an oxygen gas flow rate to an argon gas flowrate under the film formation conditions for the second In—Ga—Zn—O-basednon-single-crystal film. Specifically, the second In—Ga—Zn—O-basednon-single-crystal film is formed under a rare gas (such as argon orhelium) atmosphere (or an atmosphere including an oxygen gas at lessthan or equal to 10% and an argon gas at greater than or equal to 90%),and the first In—Ga—Zn—O-based non-single-crystal film is formed underan oxygen mixed atmosphere (an oxygen gas flow rate is higher than arare gas flow rate).

A chamber used for formation of the second In—Ga—Zn—O-basednon-single-crystal film may be the same as or different from the chamberin which the reverse sputtering has been performed.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method, and a pulsed DC sputtering method in which a bias isapplied in a pulsed manner. An RF sputtering method is mainly used inthe case where an insulating film is formed, and a DC sputtering methodis mainly used in the case where a metal film is formed.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can beformed to be stacked in the same chamber, or a film of plural kinds ofmaterials can be formed by electric discharge at the same time in thesame chamber.

In addition, there are a sputtering apparatus provided with a magnetsystem inside the chamber and used for a magnetron sputtering method,and a sputtering apparatus used for an ECR sputtering method in whichplasma generated with the use of microwaves is used without using glowdischarge.

Furthermore, as a film formation method with a sputtering method, thereare also a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringfilm formation to form a thin compound film thereof, and a biassputtering method in which voltage is also applied to a substrate duringfilm formation.

Next, the first oxide semiconductor film 131 and the second oxidesemiconductor film 136 are subjected to heat treatment for dehydrationor dehydrogenation. The first oxide semiconductor film 131 and thesecond oxide semiconductor film 136 are subjected to heat treatmentunder an atmosphere of an inert gas (such as nitrogen, helium, neon, orargon) or under reduced pressure, and then slowly cooled under an oxygenatmosphere.

The heat treatment is preferably performed at 200° C. or higher. Forexample, heat treatment is performed at 450° C. for 1 hour under anitrogen atmosphere. By this heat treatment under a nitrogen atmosphere,the resistance of the first oxide semiconductor film 131 and the secondoxide semiconductor film 136 is reduced (the carrier density isincreased, preferably to 1×10¹⁸/cm³ or higher) and the conductivitythereof is increased. Thus, a first oxide semiconductor film 133 and asecond oxide semiconductor film 137 each of whose resistance is reducedare formed (see FIG. 4C). The electrical conductivity of the first oxidesemiconductor film 133 and the second oxide semiconductor film 137 ispreferably higher than or equal to 1×10⁻¹ S/cm and lower than or equalto 1×10² S/cm.

Next, a second photolithography step is performed. A resist mask isformed, and the first oxide semiconductor film 133 and the second oxidesemiconductor film 137 are etched. For example, unnecessary portions areremoved by wet etching using a mixed solution of phosphoric acid, aceticacid, and nitric acid, so that a first oxide semiconductor layer 134 anda second oxide semiconductor layer 138 are formed. Note that etchinghere is not limited to wet etching, and dry etching may also beperformed.

As an etching gas for dry etching, a gas containing chlorine (achlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused.

Alternatively, a gas containing fluorine (a fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂);any of these gases to which a rare gas such as helium (He) or argon (Ar)is added; or the like can be used.

As the dry etching method, a parallel plate reactive ion etching (RIE)method or an inductively coupled plasma (ICP) etching method can beused. In order to etch the films into desired shapes, the etchingcondition (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) is adjusted as appropriate.

As an etchant used for wet etching, a mixed solution of phosphoric acid,acetic acid, and nitric acid, or the like can be used. In addition,ITO-07N (produced by Kanto Chemical Co., Inc.) may be used.

Furthermore, the etchant after the wet etching is removed together withthe etched material by cleaning. The waste liquid of the etchantincluding the material etched off may be purified and the material maybe reused. When a material such as indium included in the oxidesemiconductor layer is collected from the waste liquid after the etchingand reused, the resources can be efficiently used and the cost can bereduced.

In order to obtain a desired shape by etching, the etching conditions(such as an etchant, etching time, and temperature) are adjusted asappropriate depending on the material.

Next, a conductive film 132 is formed using a metal material over thefirst oxide semiconductor layer 134 and the second oxide semiconductorlayer 138 with a sputtering method or a vacuum evaporation method (seeFIG. 5B).

As the material of the conductive film 132, there are an elementselected from Al, Cr, Ta, Ti, Mo, and W, an alloy containing any ofthese elements as a component, an alloy film containing a combination ofany of these elements, and the like.

In the case where heat treatment is performed after the formation of theconductive film 132, the conductive film preferably has heat resistanceenough to withstand the heat treatment.

Next, a third photolithography step is performed. A resist mask isformed, and unnecessary portions are removed by etching, so that sourceand drain electrode layers 105 a and 105 b, a first oxide semiconductorlayer 135, source and drain regions 104 a and 104 b, and a secondterminal 122 are formed (see FIG. 5C). Wet etching or dry etching isemployed as an etching method at this time. For example, when analuminum film or an aluminum-alloy film is used as the conductive film132, wet etching using a mixed solution of phosphoric acid, acetic acid,and nitric acid can be performed. Alternatively, by wet etching using anammonia peroxide mixture (hydrogen peroxide:ammonia:water=5:2:2), theconductive film 132 may be etched to form the source and drain electrodelayers 105 a and 105 b. In the etching step, part of an exposed regionof the first oxide semiconductor layer 134 is etched, whereby the firstoxide semiconductor layer 135 is formed. Accordingly, the first oxidesemiconductor layer 135 has a region whose thickness is small betweenthe source and drain electrode layers 105 a and 105 b. In FIG. 5C, thesource and drain electrode layers 105 a and 105 b, the first oxidesemiconductor layer 135, and the source and drain regions 104 a and 104b are etched at one time by dry etching; therefore, end portions of thesource and drain electrode layers 105 a and 105 b, the first oxidesemiconductor layer 135, and the source and drain regions 104 a and 104b are aligned with one another, and a continuous structure is formed.

In the third photolithography step, the second terminal 122 which isformed using the same material as the source and drain electrode layers105 a and 105 b is left in the terminal portion. Note that the secondterminal 122 is electrically connected to a source wiring (a sourcewiring including the source and drain electrode layers 105 a and 105 b).

Further, by using a resist mask which is formed using a multi-tone maskand has regions with plural thicknesses (typically, two differentthicknesses), the number of resist masks can be reduced, resulting insimplified process and lower costs.

Next, the resist mask is removed, and a protective insulating layer 107is formed to cover the gate insulating layer 102, the first oxidesemiconductor layer 135, the source and drain regions 104 a and 104 b,and the source and drain electrode layers 105 a and 105 b. A siliconoxynitride film formed with a PCVD method is used as the protectiveinsulating layer 107. The silicon oxynitride film serving as theprotective insulating layer 107 is provided in contact with an exposedregion of the first oxide semiconductor layer 135 between the source anddrain electrode layers 105 a and 105 b, whereby the resistance of theregion of the first oxide semiconductor layer 135 which is in contactwith the protective insulating layer 107 is increased (the carrierdensity is reduced, preferably to lower than 1×10¹⁸/cm³, more preferablyto lower than or equal to 1×10¹⁴/cm³). Thus, a semiconductor layer 103including a channel formation region whose resistance is increased canbe formed.

Through the above process, a thin film transistor 170 can bemanufactured. Note that FIG. 7 is a plan view at this stage.

Next, a fourth photolithography step is performed. A resist mask isformed, and the protective insulating layer 107 and the gate insulatinglayer 102 are etched so that a contact hole 125 reaching the source ordrain electrode layer 105 b is formed. In addition, a contact hole 127reaching the second terminal 122 and a contact hole 126 reaching thefirst terminal 121 are also formed in the same etching step. FIG. 6A isa cross-sectional view at this stage.

Next, the resist mask is removed, and then a transparent conductive filmis formed. The transparent conductive film is formed using indium oxide(In₂O₃), indium oxide-tin oxide alloy (In₂O₃—SnO₂, abbreviated to ITO),or the like with a sputtering method, a vacuum evaporation method, orthe like. Such a material is etched with a hydrochloric acid-basedsolution. However, since a residue is easily generated particularly inetching ITO, indium oxide-zinc oxide alloy (In₂O₃—ZnO) may be used toimprove etching processability. Alternatively, indium oxide-tin oxidealloy including silicon oxide (an In—Sn—O-based oxide including siliconoxide) may be used. In addition, when a reflective electrode layer isused as a pixel electrode layer, it can be formed using one or morekinds of materials selected from a metal such as tungsten (W),molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium (V), niobium(Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel (Ni), titanium(Ti), platinum (Pt), aluminum (Al), copper (Cu), and silver (Ag); analloy thereof; and a nitride thereof.

Next, a fifth photolithography step is performed. A resist mask isformed, and unnecessary portions are removed by etching, so that a pixelelectrode layer 110 is formed.

In the fifth photolithography step, a storage capacitor is formed withthe capacitor wiring 108 and the pixel electrode layer 110, in which thegate insulating layer 102 and the protective insulating layer 107 in acapacitor portion are used as a dielectric.

In addition, in the fifth photolithography step, the first terminal 121and the second terminal 122 are covered with the resist mask, andtransparent conductive films 128 and 129 are left in the terminalportion. The transparent conductive films 128 and 129 function aselectrodes or wirings connected to an FPC. The transparent conductivefilm 128 formed over the first terminal 121 is a connection terminalelectrode which functions as an input terminal of a gate wiring. Thetransparent conductive film 129 formed over the second terminal 122 is aconnection terminal electrode which functions as an input terminal ofthe source wiring.

Next, the resist mask is removed. FIG. 6B is a cross-sectional view atthis stage.

Heat treatment may be performed after the formation of the protectiveinsulating layer 107 or the formation of the pixel electrode layer 110.The heat treatment may be performed at higher than or equal to 150° C.and lower than 350° C. under an air atmosphere or a nitrogen atmosphere.In the heat treatment, the semiconductor layer 103 is heated while beingin contact with the protective insulating layer 107; accordingly, theresistance of the semiconductor layer 103 is further increased, and thusimprovement and less variation in electric characteristics of thetransistor can be achieved. As for the heat treatment (preferably athigher than or equal to 150° C. and lower than 350° C.), there is noparticular limitation as long as it is performed after the formation ofthe protective insulating layer 107. When the heat treatment also servesas another step such as heat treatment in formation of a resin film orheat treatment for reducing the resistance of a transparent conductivefilm, the heat treatment can be performed without increase in the numberof steps.

Further, FIGS. 8A1 and 8A2 are a cross-sectional view of a gate wiringterminal portion at this stage and a plan view thereof, respectively.FIG. 8A1 is a cross-sectional view taken along line E1-E2 of FIG. 8A2.In FIG. 8A1, a transparent conductive film 155 formed over a protectiveinsulating film 154 is a connection terminal electrode which functionsas an input terminal. Furthermore, in the terminal portion of FIG. 8A1,a first terminal 151 formed using the same material as the gate wiringand a connection electrode layer 153 formed using the same material asthe source wiring overlap with each other with a gate insulating layer152 interposed therebetween, and are electrically connected to eachother through the transparent conductive film 155. Note that a portionwhere the transparent conductive film 128 and the first terminal 121 arein contact with each other in FIG. 6B corresponds to a portion where thetransparent conductive film 155 and the first terminal 151 are incontact with each other in FIG. 8A1.

FIGS. 8B1 and 8B2 are respectively a cross-sectional view and a planview of a source wiring terminal portion which is different from thatillustrated in FIG. 6B. FIG. 8B1 is a cross-sectional view taken alongline F1-F2 of FIG. 8B2. In FIG. 8B1, the transparent conductive film 155formed over the protective insulating film 154 is a connection terminalelectrode which functions as an input terminal. Furthermore, in theterminal portion of FIG. 8B1, an electrode layer 156 formed using thesame material as the gate wiring is located below and overlaps with asecond terminal 150 which is electrically connected to the source wiringwith the gate insulating layer 152 interposed therebetween. Theelectrode layer 156 is not electrically connected to the second terminal150, and a capacitor for preventing noise or static electricity can beformed if the potential of the electrode layer 156 is set to a potentialdifferent from that of the second terminal 150, such as floating, GND,or 0 V. The second terminal 150 is electrically connected to thetransparent conductive film 155 with the protective insulating film 154interposed therebetween.

A plurality of gate wirings, source wirings, and capacitor wirings areprovided depending on the pixel density. Also in the terminal portion,the first terminal at the same potential as the gate wiring, the secondterminal at the same potential as the source wiring, the third terminalat the same potential as the capacitor wiring, and the like are eacharranged in plurality. The number of each of the terminals may be anynumber, and the number of the terminals may be determined by apractitioner as appropriate.

Through these five photolithography steps, the storage capacitor and apixel thin film transistor portion including the thin film transistor170 which is a bottom-gate thin film transistor having a staggeredstructure can be completed using the five photomasks. By disposing thethin film transistor and the storage capacitor in each pixel of a pixelportion in which pixels are arranged in matrix, one of substrates formanufacturing an active matrix display device can be obtained. In thisspecification, such a substrate is referred to as an active matrixsubstrate for convenience.

In the case of manufacturing an active matrix liquid crystal displaydevice, an active matrix substrate and a counter substrate provided witha counter electrode are fixed to each other with a liquid crystal layerinterposed therebetween. Note that a common electrode which iselectrically connected to the counter electrode on the counter substrateis provided over the active matrix substrate, and a fourth terminalwhich is electrically connected to the common electrode is provided inthe terminal portion. The fourth terminal is a terminal for setting thecommon electrode at a fixed potential such as GND or 0 V.

Alternatively, a storage capacitor may be formed with a pixel electrodewhich overlaps with a gate wiring of an adjacent pixel, with aprotective insulating film and a gate insulating layer interposedtherebetween, without provision of the capacitor wiring.

In an active matrix liquid crystal display device, pixel electrodesarranged in matrix are driven so that a display pattern is formed on ascreen. Specifically, voltage is applied between a selected pixelelectrode and a counter electrode corresponding to the pixel electrode,so that a liquid crystal layer provided between the pixel electrode andthe counter electrode is optically modulated and this optical modulationis recognized as a display pattern by an observer.

In displaying moving images, a liquid crystal display device has aproblem in that a long response time of liquid crystal moleculesthemselves causes afterimages or blurring of moving images. In order toimprove moving image characteristics of a liquid crystal display device,there is a driving technique called black insertion by which a blackimage is displayed on the whole screen every other frame period.

Alternatively, a driving technique called double-frame rate driving maybe employed in which a vertical synchronizing frequency is 1.5 times ormore, preferably, 2 times or more as high as a usual verticalsynchronizing frequency to improve the moving-image characteristics.

Further, in order to improve moving image characteristics of a liquidcrystal display device, there is another driving technique in which asurface light source including a plurality of LED (light-emitting diode)light sources or a plurality of EL light sources is used as a backlight,and each light source included in the surface light source isindependently driven in a pulsed manner in one frame period. As thesurface light source, three or more kinds of LEDs may be used and an LEDemitting white light may be used. Since a plurality of LEDs can becontrolled independently, the light emission timing of LEDs can besynchronized with the timing at which a liquid crystal layer isoptically modulated. According to this driving technique, LEDs can bepartly turned off; therefore, an effect of reducing power consumptioncan be obtained particularly in the case of displaying an image having alarge part on which black is displayed.

By combining these driving techniques, the display characteristics of aliquid crystal display device, such as moving-image characteristics, canbe improved as compared to those of conventional liquid crystal displaydevices.

The n-channel transistor disclosed in this specification includes anoxide semiconductor film used for a channel formation region and hasfavorable dynamic characteristics; thus, it can be combined with thesedriving techniques.

In manufacturing a light-emitting display device, one electrode (alsoreferred to as a cathode) of an organic light-emitting element is set toa low power supply potential such as GND or 0 V; thus, a terminalportion is provided with a fourth terminal for setting the cathode to alow power supply potential such as GND or 0 V. Further, in manufacturinga light-emitting display device, a power supply line is provided inaddition to a source wiring and a gate wiring. Accordingly, the terminalportion is provided with a fifth terminal which is electricallyconnected to the power supply line.

In manufacturing a light-emitting display device, a partition wallincluding an organic resin layer is provided between organiclight-emitting elements in some cases. In that case, heat treatmentperformed on the organic resin layer can also serve as the heattreatment which increases the resistance of the semiconductor layer 103so that improvement and less variation in electric characteristics ofthe transistor are achieved.

The use of an oxide semiconductor for a thin film transistor leads toreduction in manufacturing cost. In particular, by the heat treatmentfor dehydration or dehydrogenation, impurities such as moisture arereduced and the purity of the oxide semiconductor film is increased.Therefore, a semiconductor device including a highly reliable thin filmtransistor having favorable electric characteristics can be manufacturedwithout using an ultrapure oxide semiconductor target or a specialsputtering apparatus in which dew point in a film-formation chamber isreduced.

Since the semiconductor layer in the channel formation region is aregion whose resistance is increased, electric characteristics of thethin film transistor are stabilized, and increase in off current or thelike can be prevented. Accordingly, a semiconductor device including thehighly reliable thin film transistor having favorable electriccharacteristics can be provided.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 5

An example will be described below, in which at least part of a drivercircuit and a thin film transistor arranged in a pixel portion areformed over the same substrate in a display device which is one exampleof a semiconductor device.

The thin film transistor to be disposed in the pixel portion is formedin accordance with any of Embodiments 1 to 4. Further, the thin filmtransistor described in any of Embodiments 1 to 4 is an n-channel TFT,and thus part of a driver circuit that can include an n-channel TFTamong driver circuits is formed over the same substrate as the thin filmtransistor of the pixel portion.

FIG. 14A illustrates an example of a block diagram of an active matrixliquid crystal display device which is an example of a semiconductordevice. The display device illustrated in FIG. 14A includes, over asubstrate 5300, a pixel portion 5301 including a plurality of pixelsthat are each provided with a display element, a scan line drivercircuit 5302 that selects each pixel, and a signal line driver circuit5303 that controls a video signal input to the selected pixel.

The pixel portion 5301 is connected to the signal line driver circuit5303 by a plurality of signal lines S1 to Sm (not illustrated) whichextend in a column direction from the signal line driver circuit 5303,and to the scan line driver circuit 5302 by a plurality of scan lines G1to Gn (not illustrated) that extend in a row direction from the scanline driver circuit 5302. The pixel portion 5301 includes a plurality ofpixels (not illustrated) arranged in matrix so as to correspond to thesignal lines S1 to Sm and the scan lines G1 to Gn. Each pixel isconnected to a signal line Sj (one of the signal lines S1 to Sm) and ascan line Gi (one of the scan lines G1 to Gn).

In addition, the thin film transistor described in any of Embodiments 1to 4 is an n-channel TFT, and a signal line driver circuit including then-channel TFT is described with reference to FIG. 15.

The signal line driver circuit illustrated in FIG. 15 includes a driverIC 5601, switch groups 5602_1 to 5602_M, a first wiring 5611, a secondwiring 5612, a third wiring 5613, and wirings 5621_1 to 5621_M. Each ofthe switch groups 5602_1 to 5602_M includes a first thin film transistor5603 a, a second thin film transistor 5603 b, and a third thin filmtransistor 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, and the third wiring 5613, and thewirings 5621_1 to 5621_M are connected to the switch groups 5602_1 to5602_M, respectively. Each of the wirings 5621_1 to 5621_M is connectedto three signal lines via the first thin film transistor 5603 a, thesecond thin film transistor 5603 b, and the third thin film transistor5603 c. For example, the wiring 5621_J of the J-th column (one of thewirings 5621_1 to 5621_M) is connected to a signal line Sj−1, a signalline Sj, and a signal line Sj+1 via the first thin film transistor 5603a, the second thin film transistor 5603 b, and the third thin filmtransistor 5603 c which are included in the switch group 5602_J.

A signal is inputted to each of the first wiring 5611, the second wiring5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed over a single crystalsubstrate. Further, the switch groups 5602_1 to 5602_M are preferablyformed over the same substrate as the pixel portion. Therefore, thedriver IC 5601 and the switch groups 5602_1 to 5602_M are preferablyconnected through an FPC or the like.

Next, operation of the signal line driver circuit illustrated in FIG. 15is described with reference to a timing chart in FIG. 16. The timingchart in FIG. 16 illustrates the case where the scan line Gi of the i-throw is selected. A selection period of the scan line Gi of the i-th rowis divided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. In addition, the signalline driver circuit in FIG. 15 operates similarly to that in FIG. 16even when a scan line of another row is selected.

Note that the timing chart in FIG. 16 illustrates the case where thewiring 5621_J of the J-th column is connected to the signal line Sj−1,the signal line Sj, and the signal line Sj+1 via the first thin filmtransistor 5603 a, the second thin film transistor 5603 b, and the thirdthin film transistor 5603 c.

Note that the timing chart in FIG. 16 illustrates timing at which thescan line Gi of the i-th row is selected, timing 5703 a at which thefirst thin film transistor 5603 a is turned on/off, timing 5703 b atwhich the second thin film transistor 5603 b is turned on/off, timing5703 c at which the third thin film transistor 5603 c is turned on/off,and a signal 5721_J inputted to the wiring 5621_J of the J-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinputted to the wirings 5621_1 to 5621_M. For example, a video signalinputted to the wiring 5621_J in the first sub-selection period T1 isinputted to the signal line Sj−1, a video signal inputted to the wiring5621_J in the second sub-selection period T2 is inputted to the signalline Sj, and a video signal inputted to the wiring 5621_J in the thirdsub-selection period T3 is inputted to the signal line Sj+1. Inaddition, the video signals inputted to the wiring 5621_J in the firstsub-selection period T1, the second sub-selection period T2, and thethird sub-selection period T3 are denoted by Data_j−1, Data_j, andData_j+1.

As illustrated in FIG. 16, in the first sub-selection period T1, thefirst thin film transistor 5603 a is turned on, and the second thin filmtransistor 5603 b and the third thin film transistor 5603 c are turnedoff. At this time, Data_j−1 inputted to the wiring 5621_J is inputted tothe signal line Sj−1 via the first thin film transistor 5603 a. In thesecond sub-selection period T2, the second thin film transistor 5603 bis turned on, and the first thin film transistor 5603 a and the thirdthin film transistor 5603 c are turned off. At this time, Data_jinputted to the wiring 5621_J is inputted to the signal line Sj via thesecond thin film transistor 5603 b. In the third sub-selection periodT3, the third thin film transistor 5603 c is turned on, and the firstthin film transistor 5603 a and the second thin film transistor 5603 bare turned off. At this time, Data_j+1 inputted to the wiring 5621_J isinputted to the signal line Sj+1 via the third thin film transistor 5603c.

As described above, in the signal line driver circuit in FIG. 15, bydividing one gate selection period into three, video signals can beinputted to three signal lines from one wiring 5621 in one gateselection period. Therefore, in the signal line driver circuit in FIG.15, the number of connections between the substrate provided with thedriver IC 5601 and the substrate provided with the pixel portion can beapproximately ⅓ of the number of signal lines. The number of connectionsis reduced to approximately ⅓ of the number of the signal lines, so thatreliability, yield, and the like of the signal line driver circuit inFIG. 15 can be improved.

Note that there is no particular limitation on the arrangement, thenumber, a driving method, and the like of the thin film transistors, aslong as one gate selection period is divided into a plurality ofsub-selection periods and video signals are inputted to a plurality ofsignal lines from one wiring in the respective sub-selection periods asillustrated in FIG. 15.

For example, when video signals are inputted to three or more signallines from one wiring in each of three or more sub-selection periods, itis only necessary to add a thin film transistor and a wiring forcontrolling the thin film transistor. Note that when one gate selectionperiod is divided into four or more sub-selection periods, onesub-selection period becomes shorter. Therefore, one gate selectionperiod is preferably divided into two or three sub-selection periods.

As another example, one gate selection period may be divided into aprecharge period Tp, the first sub-selection period T1, the secondsub-selection period T2, and the third sub-selection period T3 asillustrated in a timing chart in FIG. 17. Further, the timing chart inFIG. 17 illustrates the timing at which the scan line Gi of the i-th rowis selected, timing 5803 a at which the first thin film transistor 5603a is turned on/off, timing 5803 b at which the second thin filmtransistor 5603 b is turned on/off, timing 5803 c at which the thirdthin film transistor 5603 c is turned on/off, and a signal 5821_Jinputted to the wiring 5621_J of the J-th column. As illustrated in FIG.17, the first thin film transistor 5603 a, the second thin filmtransistor 5603 b, and the third thin film transistor 5603 c are turnedon in the precharge period Tp. At this time, precharge voltage V_(P)inputted to the wiring 5621_J is inputted to each of the signal lineSj−1, the signal line Sj, and the signal line Sj+1 via the first thinfilm transistor 5603 a, the second thin film transistor 5603 b, and thethird thin film transistor 5603 c. In the first sub-selection period T1,the first thin film transistor 5603 a is turned on, and the second thinfilm transistor 5603 b and the third thin film transistor 5603 c areturned off. At this time, Data_j−1 inputted to the wiring 5621_J isinputted to the signal line Sj−1 via the first thin film transistor 5603a. In the second sub-selection period T2, the second thin filmtransistor 5603 b is turned on, and the first thin film transistor 5603a and the third thin film transistor 5603 c are turned off. At thistime, Data_j inputted to the wiring 5621_J is inputted to the signalline Sj via the second thin film transistor 5603 b. In the thirdsub-selection period T3, the third thin film transistor 5603 c is turnedon, and the first thin film transistor 5603 a and the second thin filmtransistor 5603 b are turned off. At this time, Data_j+1 inputted to thewiring 5621_J is inputted to the signal line Sj+1 via the third thinfilm transistor 5603 c.

As described above, in the signal line driver circuit in FIG. 15 towhich the timing chart in FIG. 17 is applied, the video signal can bewritten to the pixel at high speed because the signal line can beprecharged by providing a precharge period before a sub-selectionperiod. Note that portions of FIG. 17 which are similar to those of FIG.16 are denoted by common reference numerals and detailed description ofthe same portions and portions having similar functions is omitted.

Further, a configuration of a scan line driver circuit is described. Thescan line driver circuit includes a shift register. Additionally, thescan line driver circuit may include a level shifter or a buffer in somecases. In the scan line driver circuit, when a clock signal (CLK) and astart pulse signal (SP) are inputted to the shift register, a selectionsignal is generated. The generated selection signal is buffered andamplified by the buffer, and the resulting signal is supplied to acorresponding scan line. Gate electrodes of transistors in pixels of oneline are connected to the scan line. Since the transistors in the pixelsof one line have to be turned on all at once, a buffer which can supplylarge current is used.

One mode of a shift register used for part of the scan line drivercircuit will be described with reference to FIG. 18 and FIG. 19.

FIG. 18 illustrates a circuit configuration of the shift register. Theshift register illustrated in FIG. 18 includes a plurality offlip-flops: flip-flops 5701_1 to 5701_n. The shift register is operatedwith input of a first clock signal, a second clock signal, a start pulsesignal, and a reset signal.

The connection relationship of the shift register in FIG. 18 will bedescribed. In the i-th stage flip-flop 5701_i (one of the flip-flops5701_1 to 5701_n) in the shift register in FIG. 18, a first wiring 5501illustrated in FIG. 19 is connected to a seventh wiring 5717_i−1, asecond wiring 5502 illustrated in FIG. 19 is connected to a seventhwiring 5717_i+1, a third wiring 5503 illustrated in FIG. 19 is connectedto a seventh wiring 5717_i, and a sixth wiring 5506 illustrated in FIG.19 is connected to a fifth wiring 5715.

Further, a fourth wiring 5504 illustrated in FIG. 19 is connected to asecond wiring 5712 in flip-flops of odd-numbered stages, and isconnected to a third wiring 5713 in flip-flops of even-numbered stages.A fifth wiring 5505 illustrated in FIG. 19 is connected to a fourthwiring 5714.

Note that the first wiring 5501 of the first stage flip-flop 5701_1which is illustrated in FIG. 19 is connected to a first wiring 5711.Moreover, the second wiring 5502 of the n-th stage flip-flop 5701_nwhich is illustrated in FIG. 19 is connected to a sixth wiring 5716.

Note that the first wiring 5711, the second wiring 5712, the thirdwiring 5713, and the sixth wiring 5716 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. The fourth wiring 5714 and the fifth wiring5715 may be referred to as a first power supply line and a second powersupply line, respectively.

Next, FIG. 19 illustrates details of the flip-flop shown in FIG. 18. Aflip-flop illustrated in FIG. 19 includes a first thin film transistor5571, a second thin film transistor 5572, a third thin film transistor5573, a fourth thin film transistor 5574, a fifth thin film transistor5575, a sixth thin film transistor 5576, a seventh thin film transistor5577, and an eighth thin film transistor 5578. Each of the first thinfilm transistor 5571, the second thin film transistor 5572, the thirdthin film transistor 5573, the fourth thin film transistor 5574, thefifth thin film transistor 5575, the sixth thin film transistor 5576,the seventh thin film transistor 5577, and the eighth thin filmtransistor 5578 is an n-channel transistor and is turned on when thegate-source voltage (V_(gs)) exceeds the threshold voltage (V_(th)).

Next, a connection configuration of the flip-flop illustrated in FIG. 19will be described below.

A first electrode (one of a source electrode and a drain electrode) ofthe first thin film transistor 5571 is connected to the fourth wiring5504. A second electrode (the other of the source electrode and thedrain electrode) of the first thin film transistor 5571 is connected tothe third wiring 5503.

A first electrode of the second thin film transistor 5572 is connectedto the sixth wiring 5506. A second electrode of the second thin filmtransistor 5572 is connected to the third wiring 5503.

A first electrode of the third thin film transistor 5573 is connected tothe fifth wiring 5505, and a second electrode of the third thin filmtransistor 5573 is connected to a gate electrode of the second thin filmtransistor 5572. A gate electrode of the third thin film transistor 5573is connected to the fifth wiring 5505.

A first electrode of the fourth thin film transistor 5574 is connectedto the sixth wiring 5506, and a second electrode of the fourth thin filmtransistor 5574 is connected to the gate electrode of the second thinfilm transistor 5572. A gate electrode of the fourth thin filmtransistor 5574 is connected to a gate electrode of the first thin filmtransistor 5571.

A first electrode of the fifth thin film transistor 5575 is connected tothe fifth wiring 5505, and a second electrode of the fifth thin filmtransistor 5575 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the fifth thin film transistor5575 is connected to the first wiring 5501.

A first electrode of the sixth thin film transistor 5576 is connected tothe sixth wiring 5506, and a second electrode of the sixth thin filmtransistor 5576 is connected to the gate electrode of the first thinfilm transistor 5571. A gate electrode of the sixth thin film transistor5576 is connected to the gate electrode of the second thin filmtransistor 5572.

A first electrode of the seventh thin film transistor 5577 is connectedto the sixth wiring 5506, and a second electrode of the seventh thinfilm transistor 5577 is connected to the gate electrode of the firstthin film transistor 5571. A gate electrode of the seventh thin filmtransistor 5577 is connected to the second wiring 5502. A firstelectrode of the eighth thin film transistor 5578 is connected to thesixth wiring 5506, and a second electrode of the eighth thin filmtransistor 5578 is connected to the gate electrode of the second thinfilm transistor 5572. A gate electrode of the eighth thin filmtransistor 5578 is connected to the first wiring 5501.

Note that the point at which the gate electrode of the first thin filmtransistor 5571, the gate electrode of the fourth thin film transistor5574, the second electrode of the fifth thin film transistor 5575, thesecond electrode of the sixth thin film transistor 5576, and the secondelectrode of the seventh thin film transistor 5577 are connected isreferred to as a node 5543. Further, the point at which the gateelectrode of the second thin film transistor 5572, the second electrodeof the third thin film transistor 5573, the second electrode of thefourth thin film transistor 5574, the gate electrode of the sixth thinfilm transistor 5576, and the second electrode of the eighth thin filmtransistor 5578 are connected is referred to as a node 5544.

Note that the first wiring 5501, the second wiring 5502, the thirdwiring 5503, and the fourth wiring 5504 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. The fifth wiring 5505 and the sixth wiring5506 may be referred to as a first power supply line and a second powersupply line, respectively.

In addition, the signal line driver circuit and the scan line drivercircuit can be formed using only the n-channel TFTs described in any ofEmbodiments 1 to 4. The n-channel TFT described in any of Embodiments 1to 4 have a high mobility, and thus a driving frequency of the drivercircuits can be increased. Further, since parasitic capacitance isreduced in the n-channel TFT described in any of Embodiments 1 to 4,high frequency characteristics (referred to as f characteristics) isobtained. For example, a scan line driver circuit using the n-channelTFT described in any of Embodiments 1 to 4 can operate at high speed,and thus a frame frequency can be increased and insertion of blackimages can be realized, for example.

In addition, when the channel width of the transistor in the scan linedriver circuit is increased or a plurality of scan line driver circuitsare provided, for example, higher frame frequency can be realized. Whena plurality of scan line driver circuits are provided, a scan linedriver circuit for driving scan lines of even-numbered rows is providedon one side and a scan line driver circuit for driving scan lines ofodd-numbered rows is provided on the opposite side; thus, increase inframe frequency can be realized. Furthermore, the use of the pluralityof scan line driver circuits for output of signals to the same scan lineis advantageous in increasing the size of a display device.

Further, when an active matrix light-emitting display device which is anexample of a semiconductor device is manufactured, a plurality of thinfilm transistors are arranged in at least one pixel, and thus aplurality of scan line driver circuits are preferably arranged. FIG. 14Billustrates an example of a block diagram of an active matrixlight-emitting display device.

The light-emitting display device illustrated in FIG. 14B includes, overa substrate 5400, a pixel portion 5401 including a plurality of pixelseach provided with a display element, a first scan line driver circuit5402 and a second scan line driver circuit 5404 that select each pixel,and a signal line driver circuit 5403 that controls a video signal inputto the selected pixel.

When the video signal inputted to a pixel of the light-emitting displaydevice illustrated in FIG. 14B is a digital signal, a pixel emits lightor does not emit light by switching a transistor on/off. Thus, grayscalecan be displayed using an area grayscale method or a time grayscalemethod. An area grayscale method refers to a driving method in which onepixel is divided into a plurality of sub-pixels and the respectivesub-pixels are driven independently based on video signals so thatgrayscale is displayed. Further, a time grayscale method refers to adriving method in which a period during which a pixel emits light iscontrolled so that grayscale is displayed.

Since the response speed of a light-emitting element is higher than thatof a liquid crystal element or the like, the light-emitting element ismore suitable for a time grayscale method than the liquid crystalelement. Specifically, in the case of displaying with a time grayscalemethod, one frame period is divided into a plurality of sub-frameperiods. Then, in accordance with video signals, the light-emittingelement in the pixel is brought into a light-emitting state or anon-light-emitting state in each sub-frame period. By dividing one frameperiod into a plurality of sub-frame periods, the total length of time,in which a pixel actually emits light in one frame period, can becontrolled by video signals so that grayscale can be displayed.

Note that in the example of the light-emitting display deviceillustrated in FIG. 14B, when two switching TFTs are arranged in onepixel, the first scan line driver circuit 5402 generates a signal whichis inputted to a first scan line serving as a gate wiring of one of thetwo switching TFTs, and the second scan line driver circuit 5404generates a signal which is inputted to a second scan line serving as agate wiring of the other of the two switching TFTs. However, one scanline driver circuit may generate both the signal which is inputted tothe first scan line and the signal which is inputted to the second scanline. In addition, for example, a possibility that a plurality of scanlines used to control the operation of the switching element areprovided in each pixel depending on the number of the switching TFTsincluded in one pixel. In this case, one scan line driver circuit maygenerate all signals that are inputted to the plurality of scan lines,or a plurality of scan line driver circuits may generate signals thatare inputted to the plurality of scan lines.

Also in the light-emitting display device, part of a driver circuit thatcan include an n-channel TFT among driver circuits can be formed overthe same substrate as the thin film transistors of the pixel portion.Alternatively, the signal line driver circuit and the scan line drivercircuit can be formed using only the n-channel TFTs described in any ofEmbodiments 1 to 4.

Through the above process, a highly reliable display device as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 6

When a thin film transistor is manufactured and used for a pixel portionand further for a driver circuit, a semiconductor device having adisplay function (also referred to as a display device) can bemanufactured. Furthermore, when part or whole of a driver circuit usinga thin film transistor is formed over the same substrate as a pixelportion, a system-on-panel can be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used.

Light-emitting elements include, in its category, an element whoseluminance is controlled by current or voltage, and specifically includean inorganic electroluminescent (EL) element, an organic EL element, andthe like. Furthermore, a display medium whose contrast is changed by anelectric effect, such as electronic ink, can be used.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel. An embodiment of the presentinvention also relates to an element substrate which corresponds to onemode before the display element is completed in a manufacturing processof the display device, and the element substrate is provided with a unitfor supplying current to the display element in each of a plurality ofpixels. Specifically, the element substrate may be in a state where onlya pixel electrode of the display element is formed, a state after aconductive film to be a pixel electrode is formed and before theconductive film is etched to form the pixel electrode, or any of otherstates.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Furthermore, the display device also includes the followingmodules in its category: a module to which a connector such as aflexible printed circuit (FPC), a tape automated bonding (TAB) tape, ora tape carrier package (TCP) is attached; a module having a TAB tape ora TCP at the tip of which a printed wiring board is provided; and amodule in which an integrated circuit (IC) is directly mounted on adisplay element with a chip on glass (COG) method.

The appearance and cross section of a liquid crystal display panel whichis one embodiment of a semiconductor device will be described withreference to FIGS. 10A1, 10A2 and 10B. FIGS. 10A1 and 10A2 are each aplan view of a panel in which a liquid crystal element 4013 and highlyreliable thin film transistors 4010 and 4011 each including the oxidesemiconductor layer, which are like the thin film transistor describedin Embodiment 4, which are formed over a first substrate 4001 are sealedbetween the first substrate 4001 and a second substrate 4006 with asealant 4005. FIG. 10B is a cross-sectional view taken along line M-N ofFIGS. 10A1 and 10A2.

The sealant 4005 is provided to surround a pixel portion 4002 and a scanline driver circuit 4004 that are provided over the first substrate4001. The second substrate 4006 is provided over the pixel portion 4002and the scan line driver circuit 4004. Therefore, the pixel portion 4002and the scan line driver circuit 4004 are sealed together with a liquidcrystal layer 4008, by the first substrate 4001, the sealant 4005, andthe second substrate 4006. A signal line driver circuit 4003 that isformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared is mounted in aregion different from the region surrounded by the sealant 4005 over thefirst substrate 4001.

Note that there is no particular limitation on the connection method ofa driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 10A1illustrates an example of mounting the signal line driver circuit 4003with a COG method, and FIG. 10A2 illustrates an example of mounting thesignal line driver circuit 4003 with a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 each include a plurality of thin filmtransistors, and FIG. 10B illustrates an example of the thin filmtransistor 4010 included in the pixel portion 4002 and the thin filmtransistor 4011 included in the scan line driver circuit 4004.Insulating layers 4020 and 4021 are provided over the thin filmtransistors 4010 and 4011.

As the thin film transistors 4010 and 4011, a highly reliable thin filmtransistor including oxide semiconductor layer like the thin filmtransistor described in Embodiment 4 can be employed. Alternatively, thethin film transistor described in any of Embodiments 1 to 3 may beemployed. In this embodiment, the thin film transistors 4010 and 4011are n-channel thin film transistors.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the thin film transistor 4010. A counterelectrode layer 4031 of the liquid crystal element 4013 is formed on thesecond substrate 4006. A portion where the pixel electrode layer 4030,the counter electrode layer 4031, and the liquid crystal layer 4008overlap with one another corresponds to the liquid crystal element 4013.Note that the pixel electrode layer 4030 and the counter electrode layer4031 are provided with an insulating layer 4032 and an insulating layer4033, respectively, each of which functions as an alignment film. Theliquid crystal layer 4008 is sandwiched between the pixel electrodelayer 4030 and the counter electrode layer 4031 with the insulatinglayers 4032 and 4033 interposed therebetween.

Note that the first substrate 4001 and the second substrate 4006 can bemade of glass, metal (typically, stainless steel), ceramic, or plastic.As plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinylfluoride (PVF) film, a polyester film, or an acrylic resin film can beused. Alternatively, a sheet with a structure in which an aluminum foilis sandwiched between PVF films or polyester films can be used.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the distance (a cell gap) between the pixel electrode layer 4030and the counter electrode layer 4031. Note that a spherical spacer mayalso be used. The counter electrode layer 4031 is electrically connectedto a common potential line provided over the same substrate as the thinfilm transistor 4010. With the use of the common connection portion, thecounter electrode layer 4031 can be electrically connected to the commonpotential line through conductive particles provided between the pair ofsubstrates. Note that the conductive particles are contained in thesealant 4005.

Alternatively, liquid crystal showing a blue phase for which analignment film is unnecessary may be used. A blue phase is one of theliquid crystal phases, which is generated just before a cholestericphase changes into an isotropic phase while temperature of cholestericliquid crystal is increased. Since the blue phase is only generatedwithin a narrow range of temperatures, a liquid crystal compositioncontaining a chiral agent at 5 wt % or more is used for the liquidcrystal layer 4008 in order to improve the temperature range. The liquidcrystal composition which includes liquid crystal exhibiting a bluephase and a chiral agent has a short response speed of 1 msec or less,has optical isotropy, which makes the alignment process unneeded, andhas a small viewing angle dependence.

An embodiment of the present invention can also be applied to areflective liquid crystal display device or a semi-transmissive liquidcrystal display device, in addition to a transmissive liquid crystaldisplay device.

An example of the liquid crystal display device is described in which apolarizing plate is provided on the outer surface of the substrate (onthe viewer side) and a coloring layer and an electrode layer used for adisplay element are provided on the inner surface of the substrate inthis order; however, the polarizing plate may be provided on the innersurface of the substrate. The stacked-layer structure of the polarizingplate and the coloring layer is not limited to that described in thisembodiment and may be set as appropriate in a manner that depends onmaterials of the polarizing plate and the coloring layer or conditionsof manufacturing steps. Furthermore, a light-blocking film serving as ablack matrix may be provided.

In the thin film transistors 4010 and 4011, the insulating layer 4020 isformed as a protective insulating film so as to be in contact with asemiconductor layer including a channel formation region. The insulatinglayer 4020 may be formed using a material and a method similar to thoseof the oxide insulating film 407 described in Embodiment 1. In addition,in order to reduce the surface roughness of the thin film transistors,the thin film transistors are covered with the insulating layer 4021serving as a planarizing insulating film.

In this embodiment, the insulating layer 4020 having a stacked-layerstructure is formed as the protective film. As a first layer of theinsulating layer 4020, a silicon oxide film is formed with a sputteringmethod. The use of the silicon oxide film as the protective film has theeffect of preventing a hillock of an aluminum film used for the sourceand drain electrode layers.

An insulating layer is formed as a second layer of the protective film.As a second layer of the insulating layer 4020, a silicon nitride filmis formed with a sputtering method. The use of the silicon nitride filmas the protective film can prevent mobile ions such as sodium ions fromentering a semiconductor region, thereby suppressing variations inelectric characteristics of the TFTs.

After the protective film is formed, heat treatment (at 300° C. or less)may be performed under a nitrogen atmosphere or an air atmosphere.

The insulating layer 4021 is formed as the planarizing insulating film.As the insulating layer 4021, an organic material having heat resistancesuch as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can beused. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating films formed using any of these materials.

Note that a siloxane-based resin is a resin formed from a siloxane-basedmaterial as a starting material and having a Si—O—Si bond. Thesiloxane-based resin may include an organic group (e.g., an alkyl groupor an aryl group) or a fluoro group as a substituent. The organic groupmay include a fluoro group.

There is no particular limitation on the method for forming theinsulating layer 4021, and any of the following can be used depending ona material thereof: a method such as a sputtering method, an SOG method,spin coating, dipping, spray coating, or a droplet discharging method(e.g., an ink-jet method, screen printing, or offset printing); a toolsuch as doctor knife, roll coater, curtain coater, or knife coater; orthe like. The baking step of the insulating layer 4021 also serves asthe annealing step of the semiconductor layer, whereby a semiconductordevice can be manufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

A conductive composition containing a conductive macromolecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition preferably has a sheetresistance of 10000 ohms per square or less and a transmittance of 70%or more at a wavelength of 550 nm. Furthermore, the resistivity of theconductive macromolecule contained in the conductive composition ispreferably 0.1 Ω·cm or less.

As the conductive macromolecule, a so-called π-electron conjugatedconductive polymer can be used. For example, it is possible to usepolyaniline or a derivative thereof, polypyrrole or a derivativethereof, polythiophene or a derivative thereof, or a copolymer of two ormore kinds of them.

In addition, a variety of signals and potentials are supplied from anFPC 4018 to the signal line driver circuit 4003 that is formedseparately, and the scan line driver circuit 4004 or the pixel portion4002.

A connection terminal electrode 4015 is formed from the same conductivefilm as the pixel electrode layer 4030 included in the liquid crystalelement 4013, and a terminal electrode 4016 is formed from the sameconductive film as source and drain electrode layers of the thin filmtransistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Note that FIGS. 10A1, 10A2 and 10B illustrate an example in which thesignal line driver circuit 4003 is formed separately and mounted on thefirst substrate 4001; however, the present invention is not limited tothis structure. The scan line driver circuit may be separately formedand then mounted, or only part of the signal line driver circuit or partof the scan line driver circuit may be separately formed and thenmounted.

FIG. 20 illustrates an example of a liquid crystal display module whichis formed as a semiconductor device by using a TFT substrate 2600manufactured in accordance with the manufacturing method disclosed inthis specification.

FIG. 20 illustrates an example of a liquid crystal display module, inwhich the TFT substrate 2600 and a counter substrate 2601 are bonded toeach other with a sealant 2602, and a pixel portion 2603 including a TFTor the like, a display element 2604 including a liquid crystal layer,and a coloring layer 2605 are provided between the substrates to form adisplay region. The coloring layer 2605 is necessary to perform colordisplay. In the case of the RGB system, respective coloring layerscorresponding to colors of red, green, and blue are provided forrespective pixels. Polarizing plates 2606 and 2607 and a diffusion plate2613 are provided outside the TFT substrate 2600 and the countersubstrate 2601. A light source includes a cold cathode tube 2610 and areflective plate 2611. A circuit board 2612 is connected to a wiringcircuit portion 2608 of the TFT substrate 2600 through a flexible wiringboard 2609 and includes an external circuit such as a control circuit ora power source circuit. The polarizing plate and the liquid crystallayer may be stacked with a retardation plate interposed therebetween.

For the liquid crystal display module, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an axially symmetric aligned micro-cell (ASM)mode, an optical compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, or the like can be used.

Through the above process, a highly reliable liquid crystal displaypanel as a semiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 7

An example of electronic paper will be described as a semiconductordevice.

The semiconductor device can be used for electronic paper that driveselectronic using an element electrically connected to a switchingelement. The electronic paper is also referred to as an electrophoreticdisplay device (an electrophoretic display) and is advantageous in thatit has the same level of readability as plain paper, it has lower powerconsumption than other display devices, and it can be made thin andlightweight.

Electrophoretic displays can have various modes. Electrophoreticdisplays contain a plurality of microcapsules dispersed in a solvent ora solute, and each microcapsule contains first particles which arepositively charged and second particles which are negatively charged. Byapplication of an electric field to the microcapsules, the particles inthe microcapsules move in opposite directions to each other and only thecolor of the particles gathering on one side is displayed. Note that thefirst particles and the second particles each contain pigment and do notmove without an electric field. Moreover, the first particles and thesecond particles have different colors (which may be colorless).

An electrophoretic display is thus a display that utilizes a so-calleddielectrophoretic effect by which a substance having a high dielectricconstant moves to a high-electric field region. An electrophoreticdisplay does not need to use a polarizing plate which is required in aliquid crystal display device.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

In addition, when a plurality of the microcapsules are arranged asappropriate over an active matrix substrate so as to be interposedbetween two electrodes, an active matrix display device can becompleted, and thus display can be performed by application of anelectric field to the microcapsules. For example, the active matrixsubstrate obtained using the thin film transistor described in any ofEmbodiments 1 to 4 can be used.

Note that the first particles and the second particles in themicrocapsules may each be formed using a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed using a composite material of any ofthese.

FIG. 9 illustrates active matrix electronic paper as an example of thesemiconductor device. A thin film transistor 581 used for thesemiconductor device can be manufactured in a manner similar to that ofthe thin film transistor described in Embodiment 1 and is a highlyreliable thin film transistor including an oxide semiconductor layer.The thin film transistor described in any of Embodiments 2 to 4 can alsobe used as the thin film transistor 581 of this embodiment.

The electronic paper in FIG. 9 is an example of a display device using atwisting ball display system. The twisting ball display system refers toa method in which spherical particles each colored in black and whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer to control orientation of the sphericalparticles, so that display is performed.

The thin film transistor 581 sealed between a substrate 580 and asubstrate 596 is a thin film transistor having a bottom-gate structureand is covered with an insulating film 583 that is in contact with thesemiconductor layer. A source or drain electrode layer of the thin filmtransistor 581 is in contact with a first electrode layer 587 through anopening formed in the insulating film 583 and an insulating layer 585,whereby the thin film transistor 581 is electrically connected to thefirst electrode layer 587. Between the first electrode layer 587 and asecond electrode layer 588, spherical particles 589 each having a blackregion 590 a, a white region 590 b, and a cavity 594 which is filledwith liquid around the black region 590 a and the white region 590 b areprovided. A space around the spherical particles 589 is filled with afiller 595 such as a resin (see FIG. 9). The first electrode layer 587corresponds to a pixel electrode, and the second electrode layer 588corresponds to a common electrode. The second electrode layer 588 iselectrically connected to a common potential line provided over the samesubstrate 580 as the thin film transistor 581. With the use of a commonconnection portion, the second electrode layer 588 can be electricallyconnected to the common potential line through conductive particlesprovided between the substrate 580 and the substrate 596.

Instead of the twisting ball, an electrophoretic element can also beused. A microcapsule having a diameter of approximately 10 μm to 200 μmin which transparent liquid, positively-charged white microparticles,and negatively-charged black microparticles are encapsulated is used. Inthe microcapsule which is provided between the first electrode layer andthe second electrode layer, when an electric field is applied by thefirst electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element and isgenerally called electronic paper. The electrophoretic display elementhas higher reflectance than a liquid crystal display element, and thusan auxiliary light is unnecessary, power consumption is low, and adisplay portion can be recognized even in a dim place. In addition, evenwhen power is not supplied to the display portion, an image which hasbeen displayed once can be maintained. Accordingly, a displayed imagecan be stored even if a semiconductor device having a display function(which may be referred to simply as a display device or a semiconductordevice provided with a display device) is distanced from an electricwave source.

Through above process, highly reliable electronic paper as asemiconductor device can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 8

An example of a light-emitting display device will be described as thesemiconductor device. As a display element included in the displaydevice, a light-emitting element utilizing electroluminescence isdescribed in this embodiment. Light-emitting elements utilizingelectroluminescence are classified according to whether a light-emittingmaterial is an organic compound or an inorganic compound. In general,the former is referred to as an organic EL element, and the latter isreferred to as an inorganic EL element.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. Then, the carriers (electrons and holes) recombine,so that the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission which utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade in this embodiment using an organic EL element as a light-emittingelement.

FIG. 12 illustrates an example of a pixel configuration to which digitaltime grayscale driving can be applied as an example of the semiconductordevice.

The configuration and operation of a pixel to which digital timegrayscale driving can be applied will be described. An example isdescribed in this embodiment in which one pixel includes two n-channeltransistors using an oxide semiconductor layer in a channel formationregion.

A pixel 6400 includes a switching transistor 6401, a driving transistor6402, a light-emitting element 6404, and a capacitor 6403. In theswitching transistor 6401, a gate thereof is connected to a scan line6406, a first electrode thereof (one of source and drain electrodes) isconnected to a signal line 6405, and a second electrode thereof (theother of the source and drain electrodes) is connected to a gate of thedriving transistor 6402. In the driving transistor 6402, the gatethereof is connected to a power supply line 6407 through the capacitor6403, a first electrode thereof is connected to the power supply line6407, and a second electrode thereof is connected to a first electrode(pixel electrode) of the light-emitting element 6404. A second electrodeof the light-emitting element 6404 corresponds to a common electrode6408. The common electrode 6408 is electrically connected to a commonpotential line provided over the same substrate.

Note that the second electrode (common electrode 6408) of thelight-emitting element 6404 is set to a low power supply potential. Notethat the low power supply potential is a potential satisfying the lowpower supply potential<a high power supply potential with reference tothe high power supply potential that is set on the power supply line6407. As the low power supply potential, GND, 0 V, or the like may beemployed, for example. The difference between the high power supplypotential and the low power supply potential is applied to thelight-emitting element 6404 so that current flows through thelight-emitting element 6404, whereby the light-emitting element 6404emits light. Thus, each potential is set so that the difference betweenthe high power supply potential and the low power supply potential isgreater than or equal to a forward threshold voltage of thelight-emitting element 6404.

When the gate capacitance of the driving transistor 6402 is used as asubstitute for the capacitor 6403, the capacitor 6403 can be omitted.The gate capacitance of the driving transistor 6402 may be formedbetween the channel region and the gate electrode.

In the case of using a voltage-input voltage driving method, a videosignal is inputted to the gate of the driving transistor 6402 so thatthe driving transistor 6402 is in either of two states of beingsufficiently turned on and turned off. That is, the driving transistor6402 operates in a linear region, and thus a voltage higher than thevoltage of the power supply line 6407 is applied to the gate of thedriving transistor 6402. Note that a voltage higher than or equal to thefollowing is applied to the signal line 6405: power supply linevoltage+V_(th) of the driving transistor 6402.

In the case of performing analog grayscale driving instead of digitaltime grayscale driving, the same pixel configuration as FIG. 12 can beemployed by inputting signals in a different way.

In the case of performing analog grayscale driving, voltage higher thanor equal to the following is applied to the gate of the drivingtransistor 6402: forward voltage of the light-emitting element6404+V_(th) of the driving transistor 6402. The forward voltage of thelight-emitting element 6404 refers to voltage to obtain a desiredluminance, and includes at least forward threshold voltage. By input ofa video signal which enables the driving transistor 6402 to operate in asaturation region, it is possible to feed current to the light-emittingelement 6404. In order that the driving transistor 6402 can operate inthe saturation region, the potential of the power supply line 6407 isset higher than a gate potential of the driving transistor 6402. When ananalog video signal is used, it is possible to feed current to thelight-emitting element 6404 in accordance with the video signal andperform analog grayscale driving.

Note that the pixel configuration is not limited to that illustrated inFIG. 12. For example, the pixel illustrated in FIG. 12 may furtherinclude a switch, a resistor, a capacitor, a transistor, a logiccircuit, or the like.

Next, structures of the light-emitting element will be described withreference to FIGS. 13A to 13C. A cross-sectional structure of a pixelwill be described by taking an n-channel driving TFT as an example.Driving TFTs 7001, 7011, and 7021 used for semiconductor devicesillustrated in FIGS. 13A, 13B, and 13C, respectively, can bemanufactured in a manner similar to that of the thin film transistordescribed in Embodiment 1 and are highly reliable thin film transistorseach including an oxide semiconductor layer. Alternatively, the thinfilm transistors described in any of Embodiment 2 to 4 can be employedas the driving TFTs 7001, 7011, and 7021.

In order to extract light emitted from the light-emitting element, atleast one of the anode and the cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure inwhich light is extracted through the surface opposite to the substrate,a bottom emission structure in which light is extracted through thesurface on the substrate side, or a dual emission structure in whichlight is extracted through the surface opposite to the substrate and thesurface on the substrate side. The pixel configuration can be applied toa light-emitting element having any of these emission structures.

A light-emitting element having a top emission structure will bedescribed with reference to FIG. 13A.

FIG. 13A is a cross-sectional view of a pixel in the case where thedriving TFT 7001 is an n-channel TFT and light is emitted from alight-emitting element 7002 to an anode 7005 side. In FIG. 13A, acathode 7003 of the light-emitting element 7002 is electricallyconnected to the driving TFT 7001, and a light-emitting layer 7004 andthe anode 7005 are stacked in this order over the cathode 7003. Thecathode 7003 can be formed using a variety of conductive materials aslong as they have a low work function and reflect light. For example,Ca, Al, MgAg, AlLi, or the like is preferably used. The light-emittinglayer 7004 may be formed as a single layer or a plurality of layersstacked. When the light-emitting layer 7004 is formed as a plurality oflayers, the light-emitting layer 7004 is formed by stacking anelectron-injection layer, an electron-transport layer, a light-emittinglayer, a hole-transport layer, and a hole-injection layer in this orderover the cathode 7003. Note that not all of these layers need to beprovided. The anode 7005 may be formed using a light-transmittingconductive material such as indium oxide including tungsten oxide,indium zinc oxide including tungsten oxide, indium oxide includingtitanium oxide, indium tin oxide including titanium oxide, indium tinoxide (hereinafter referred to as ITO), indium zinc oxide, or indium tinoxide to which silicon oxide is added.

The light-emitting element 7002 corresponds to a region where thelight-emitting layer 7004 is sandwiched between the cathode 7003 and theanode 7005. In the case of the pixel illustrated in FIG. 13A, light isemitted from the light-emitting element 7002 to the anode 7005 side asindicated by an arrow.

Next, a light-emitting element having a bottom emission structure willbe described with reference to FIG. 13B. FIG. 13B is a cross-sectionalview of a pixel in the case where the driving TFT 7011 is an n-channelTFT and light is emitted from a light-emitting element 7012 to a cathode7013 side. In FIG. 13B, the cathode 7013 of the light-emitting element7012 is formed over a light-transmitting conductive film 7017 which iselectrically connected to the driving TFT 7011, and a light-emittinglayer 7014 and an anode 7015 are stacked in this order over the cathode7013. Note that a light-blocking film 7016 for reflecting or blockinglight may be formed so as to cover the anode 7015 when the anode 7015has a light-transmitting property. As in the case of FIG. 13A, thecathode 7013 can be formed using a variety of conductive materials aslong as they have a low work function. Note that the cathode 7013 isformed to have a thickness that can transmit light (preferably,approximately 5 nm to 30 nm). For example, an aluminum film with athickness of 20 nm can be used as the cathode 7013. As in the case ofFIG. 13A, the light-emitting layer 7014 may be formed using either asingle layer or a plurality of layers stacked. The anode 7015 is notrequired to transmit light, but can be formed using a light-transmittingconductive material as in the case of FIG. 13A. As the light-blockingfilm 7016, a metal which reflects light can be used for example;however, the light-blocking film 7016 is not limited to a metal film.For example, a resin to which black pigments are added can also be used.

The light-emitting element 7012 corresponds to a region where thelight-emitting layer 7014 is sandwiched between the cathode 7013 and theanode 7015. In the case of the pixel illustrated in FIG. 13B, light isemitted from the light-emitting element 7012 to the cathode 7013 side asindicated by an arrow.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 13C. In FIG. 13C, a cathode 7023 of alight-emitting element 7022 is formed over a light-transmittingconductive film 7027 which is electrically connected to the driving TFT7021, and a light-emitting layer 7024 and an anode 7025 are stacked inthis order over the cathode 7023. As in the case of FIG. 13A, thecathode 7023 can be formed using a variety of conductive materials aslong as they have a low work function. Note that the cathode 7023 isformed to have a thickness that can transmit light. For example, analuminum film with a thickness of 20 nm can be used as the cathode 7023.As in FIG. 13A, the light-emitting layer 7024 may be formed using eithera single layer or a plurality of layers stacked. The anode 7025 can beformed using a light-transmitting conductive material as in the case ofFIG. 13A.

The light-emitting element 7022 corresponds to a region where thecathode 7023, the light-emitting layer 7024, and the anode 7025 overlapwith one another. In the case of the pixel illustrated in FIG. 13C,light is emitted from the light-emitting element 7022 to both the anode7025 side and the cathode 7023 side as indicated by arrows.

Although an organic EL element is described in this embodiment as alight-emitting element, an inorganic EL element can also be provided asa light-emitting element.

Note that although the example is described in which a thin filmtransistor (a driving TFT) which controls the driving of alight-emitting element is electrically connected to the light-emittingelement, a structure may be employed in which a TFT for current controlis connected between the driving TFT and the light-emitting element.

Note that the structure of the semiconductor device described in thisembodiment is not limited to those illustrated in FIGS. 13A to 13C andcan be modified in various ways based on the spirit of techniquesdisclosed in this specification.

Next, the appearance and cross section of a light-emitting display panel(also referred to as a light-emitting panel), which is one embodiment ofthe semiconductor device, will be described with reference to FIGS. 11Aand 11B. FIG. 11A is a plan view of a panel in which a thin filmtransistor and a light-emitting element formed over a first substrateare sealed between the first substrate and a second substrate with asealant. FIG. 11B is a cross-sectional view taken along line H-I of FIG.11A.

A sealant 4505 is provided to surround a pixel portion 4502, signal linedriver circuits 4503 a and 4503 b, and scan line driver circuits 4504 aand 4504 b, which are provided over a first substrate 4501. In addition,a second substrate 4506 is provided over the pixel portion 4502, thesignal line driver circuits 4503 a and 4503 b, and the scan line drivercircuits 4504 a and 4504 b. Accordingly, the pixel portion 4502, thesignal line driver circuits 4503 a and 4503 b, and the scan line drivercircuits 4504 a and 4504 b are sealed together with a filler 4507, bythe first substrate 4501, the sealant 4505, and the second substrate4506. It is preferable that packaging (sealing) be thus performed with aprotective film (such as a bonding film or an ultraviolet curable resinfilm) or a cover material with high air-tightness and littledegasification so that the display device is not thus exposed to theoutside air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors,and a thin film transistor 4510 included in the pixel portion 4502 and athin film transistor 4509 included in the signal line driver circuit4503 a are illustrated as an example in FIG. 11B.

As the thin film transistors 4509 and 4510, the highly reliable thinfilm transistor including an oxide semiconductor layer, which isdescribed in Embodiment 3, can be employed. Alternatively, the thin filmtransistor described in any of Embodiments 1 to 4 can be employed. Inthis embodiment, the thin film transistors 4509 and 4510 are n-channelthin film transistors.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 that is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a source ordrain electrode layer of the thin film transistor 4510. Note that astructure of the light-emitting element 4511 is not limited to thestacked-layer structure described in this embodiment, which includes thefirst electrode layer 4517, an electroluminescent layer 4512, and asecond electrode layer 4513. The structure of the light-emitting element4511 can be changed as appropriate in a manner that depends on thedirection in which light is extracted from the light-emitting element4511, for example.

A partition wall 4520 is formed using an organic resin film, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition wall 4520 be formed using a photosensitivematerial to have an opening over the first electrode layer 4517 so thata sidewall of the opening is formed as an inclined surface withcontinuous curvature.

The electroluminescent layer 4512 may be formed as a single layer or aplurality of layers stacked.

A protective film may be formed over the second electrode layer 4513 andthe partition wall 4520 in order to prevent oxygen, hydrogen, moisture,carbon dioxide, or the like from entering the light-emitting element4511. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed.

A variety of signals and potentials are supplied from FPCs 4518 a and4518 b to the signal line driver circuits 4503 a and 4503 b, the scanline driver circuits 4504 a and 4504 b, or the pixel portion 4502.

A connection terminal electrode 4515 is formed from the same conductivefilm as the first electrode layer 4517 included in the light-emittingelement 4511, and a terminal electrode 4516 is formed from the sameconductive film as source and drain electrode layers included in thethin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal of the FPC 4518 a through an anisotropic conductive film 4519.

The second substrate 4506 located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, polyvinyl chloride (PVC), acrylic, polyimide, anepoxy resin, a silicone resin, polyvinyl butyral (PVB), or ethylenevinyl acetate (EVA) can be used. For example, nitrogen may be used forthe filler.

If needed, an optical film such as a polarizing plate, a circularlypolarizing plate (including an elliptically polarizing plate), aretardation plate (a quarter-wave plate or a half-wave plate), or acolor filter may be provided as appropriate on a light-emitting surfaceof the light-emitting element. Furthermore, the polarizing plate or thecircularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so that the glareis reduced can be performed.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be mounted as driver circuitsformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared. Alternatively,only the signal line driver circuits or part thereof, or only the scanline driver circuits or part thereof may be separately formed andmounted. The present invention is not limited to the structureillustrated in FIGS. 11A and 11B.

Through the above process, a highly reliable light-emitting displaypanel (light-emitting panel) as a semiconductor device can bemanufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 9

A semiconductor device disclosed in this specification can be applied toelectronic paper. Electronic paper can be used for electronic appliancesof a variety of fields as long as they can display data. For example,electronic paper can be applied to an electronic book (electronic book)reader, a poster, an advertisement in a vehicle such as a train, ordisplays of various cards such as a credit card. An example of theelectronic appliances is illustrated in FIG. 22.

FIG. 22 illustrates an example of an electronic book reader 2700. Forexample, the electronic book reader 2700 includes two housings, ahousing 2701 and a housing 2703. The housing 2701 and the housing 2703are combined with a hinge 2711 so that the electronic book reader 2700can be opened and closed with the hinge 2711 as an axis. With such astructure, the electronic book reader 2700 can operate like a paperbook.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, text can bedisplayed on a display portion on the right side (the display portion2705 in FIG. 22) and images can be displayed on a display portion on theleft side (the display portion 2707 in FIG. 22).

FIG. 22 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, and the like may be provided onthe same surface as the display portion of the housing. Furthermore, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal that can be connected to various cables such as an AC adapterand a USB cable, or the like), a recording medium insertion portion, orthe like may be provided on the back surface or the side surface of thehousing. Moreover, the electronic book reader 2700 may have a functionof an electronic dictionary.

Further, the electronic book reader 2700 may send and receiveinformation wirelessly. Through wireless communication, desired bookdata or the like can be purchased and downloaded from an electronic bookserver.

Embodiment 10

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including amusement machines).Examples of electronic appliances include television sets (also referredto as televisions or television receivers), monitors of computers or thelike, cameras such as digital cameras or digital video cameras, digitalphoto frames, cellular phones (also referred to as mobile phones ormobile phone sets), portable game consoles, portable informationterminals, audio reproducing devices, large-sized game machines such aspachinko machines, and the like.

FIG. 23A illustrates an example of a television set 9600. In thetelevision set 9600, a display portion 9603 is incorporated in a housing9601. Images can be displayed on the display portion 9603. Here, thehousing 9601 is supported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610 so that an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 which display data outputted from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the receiver, a general television broadcast can bereceived. Furthermore, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver, between receivers, or the like) datacommunication can be performed.

FIG. 23B illustrates an example of a digital photo frame 9700. Forexample, in the digital photo frame 9700, a display portion 9703 isincorporated in a housing 9701. Various images can be displayed on thedisplay portion 9703. For example, the display portion 9703 can displayimage data taken with a digital camera or the like to function as anormal photo frame.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection portion (a USB terminal, a terminal thatcan be connected to various cables such as a USB cable, or the like), arecording medium insertion portion, and the like. Although they may beprovided on the same surface as the display portion 9703, it ispreferable to provide them on the side surface or the back surfacebecause the design thereof is improved. For example, a memory in whichimage data taken with a digital camera is stored is inserted in therecording medium insertion portion of the digital photo frame 9700,whereby the image data can be displayed on the display portion 9703.

The digital photo frame 9700 may send and receive informationwirelessly. Through wireless communication, desired image data can bedownloaded to be displayed.

FIG. 24A illustrates a portable amusement machine including twohousings, a housing 9881 and a housing 9891. The housings 9881 and 9891are connected with a connection portion 9893 so as to be opened andclosed. A display portion 9882 and a display portion 9883 areincorporated in the housing 9881 and the housing 9891, respectively. Inaddition, the portable amusement machine illustrated in FIG. 24Aincludes a speaker portion 9884, a recording medium insertion portion9886, an LED lamp 9890, an input unit (an operation key 9885, aconnection terminal 9887, a sensor 9888 (a sensor having a function ofmeasuring force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,current, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays), and a microphone 9889),and the like. It is needless to say that the structure of the portableamusement machine is not limited to the above, and other structuresprovided with at least a semiconductor device disclosed in thisspecification may be employed. The portable amusement machine mayinclude other accessory equipment as appropriate. The portable amusementmachine illustrated in FIG. 24A has a function of reading a program ordata stored in a recording medium to display it on the display portion,and a function of sharing information with another portable amusementmachine by wireless communication. The portable amusement machineillustrated in FIG. 24A can have various functions without limitation tothe above.

FIG. 24B illustrates an example of a slot machine 9900 which is alarge-sized amusement machine. In the slot machine 9900, a displayportion 9903 is incorporated in a housing 9901. In addition, the slotmachine 9900 includes an operation unit such as a start lever or a stopswitch, a coin slot, a speaker, and the like. It is needless to say thatthe structure of the slot machine 9900 is not limited to the above, andother structures provided with at least a semiconductor device disclosedin this specification may be employed. The slot machine 9900 may includeother accessory equipment as appropriate.

FIG. 25A is a perspective view illustrating an example of a portablecomputer.

In the portable computer in FIG. 25A, a top housing 9301 having adisplay portion 9303 and a bottom housing 9302 having a keyboard 9304can overlap with each other by closing a hinge unit which connects thetop housing 9301 and the bottom housing 9302. The portable computer inFIG. 25A can be convenient for carrying, and in the case of using thekeyboard for input, the hinge unit is opened so that the user can inputlooking at the display portion 9303.

The bottom housing 9302 includes a pointing device 9306 with which inputcan be performed, in addition to the keyboard 9304. Further, when thedisplay portion 9303 is a touch input panel, input can be performed bytouching part of the display portion. The bottom housing 9302 includesan arithmetic function portion such as a CPU or hard disk. In addition,the bottom housing 9302 includes another device, for example, anexternal connection port 9305 into which a communication cableconformable to communication standards of a USB is inserted.

The top housing 9301 further includes a display portion 9307 which canbe stored in the top housing 9301 by being slid therein. Thus, a largedisplay screen can be realized. In addition, the user can adjust theorientation of a screen of the storable display portion 9307. When thestorable display portion 9307 is a touch input panel, input can beperformed by touching part of the storable display portion.

The display portion 9303 or the storable display portion 9307 is formedusing an image display device of a liquid crystal display panel, alight-emitting display panel such as an organic light-emitting elementor an inorganic light-emitting element, or the like.

In addition, the portable computer in FIG. 25A, which can be providedwith a receiver and the like, can receive a television broadcast todisplay an image on the display portion. While the hinge unit whichconnects the top housing 9301 and the bottom housing 9302 is keptclosed, the whole screen of the display portion 9307 is exposed bysliding the display portion 9307 out and the angle of the screen isadjusted; thus, the user can watch a television broadcast. In this case,the hinge unit is not opened and display is not performed on the displayportion 9303. In addition, start up of only a circuit which displays thetelevision broadcast is performed. Therefore, power consumption can beminimized, which is advantageous for the portable computer whose batterycapacity is limited.

FIG. 25B is a perspective view illustrating an example of a cellularphone that the user can wear on the wrist like a wristwatch.

This cellular phone includes a main body which includes a battery and acommunication device having at least a telephone function; a bandportion 9204 which enables the main body to be wore on the wrist; anadjusting portion 9205 which adjusts the band portion 9204 to fit thewrist; a display portion 9201; a speaker 9207; and a microphone 9208.

In addition, the main body includes an operation switch 9203. Theoperation switch 9203 serves, for example, as a switch for starting aprogram for the Internet when the switch is pushed, in addition toserving as a switch for turning on a power source, a switch for shiftinga display, a switch for instructing to start taking images, or the like,and can be used so as to correspond to each function.

Input to this cellular phone is operated by touching the display portion9201 with a finger, an input pen, or the like, by operating theoperation switch 9203, or by inputting voice into the microphone 9208.Note that displayed buttons 9202 which are displayed on the displayportion 9201 are illustrated in FIG. 25B. Input can be performed bytouching the displayed buttons 9202 with a finger or the like.

Further, the main body includes a camera portion 9206 including an imagepick-up unit having a function of converting an image of an object,which is formed through a camera lens, to an electronic image signal.Note that the camera portion is not necessarily provided.

The cellular phone illustrated in FIG. 25B, which can be provided with areceiver of a television broadcast and the like, can display an image onthe display portion 9201 by receiving a television broadcast. Inaddition, the cellular phone illustrated in FIG. 25B may be providedwith a storage device and the like such as a memory, and thus can recorda television broadcast in the memory. The cellular phone illustrated inFIG. 25B may have a function of collecting location information, such asthe GPS.

The display portion 9201 is formed using an image display device of aliquid crystal display panel, a light-emitting display panel such as anorganic light-emitting element or an inorganic light-emitting element,or the like. The cellular phone illustrated in FIG. 25B is compact andlightweight and thus has limited battery capacity. Therefore, a panelwhich can be driven with low power consumption is preferably used as adisplay device for the display portion 9201.

Note that FIG. 25B illustrates the electronic appliance which is worn onthe wrist; however, this embodiment is not limited thereto as long as aportable shape is employed.

Example 1

In this example, in an oxide semiconductor layer including a regionhaving high oxygen density and a region having low oxygen density, thesimulation result thereof in change of the oxygen density before andafter heat treatment will be described with reference to FIG. 34 andFIG. 21. As software for the simulation, Materials Explorer 5.0manufactured by Fujitsu Limited was used.

FIG. 34 illustrates a model of an oxide semiconductor layer which wasused for the simulation. Here, a structure in which a layer 703 havinglow oxygen density and a layer 705 having high oxygen density werestacked was employed for an oxide semiconductor layer 701.

For the layer 703 having low oxygen density, an amorphous structure wasemployed in which the numbers of In atoms, Ga atoms, and Zn atoms wereeach 15 and the number of O atoms was 54.

In addition, for the layer 705 having high oxygen density, an amorphousstructure was employed in which the numbers of In atoms, Ga atoms, andZn atoms were each 15 and the number of O atoms was 66.

Moreover, the density of the oxide semiconductor layer 701 was set at5.9 g/cm³.

Next, the classical molecular dynamics (MD) simulation was performed onthe oxide semiconductor layer 701 under conditions of NVT ensemble and atemperature of 250° C. The time step was set at 0.2 fs, and the totalsimulation time was set at 200 ps. In addition, Born-Mayer-Hugginspotential was used for the potentials of metal-oxygen bonding andoxygen-oxygen bonding. Moreover, movement of atoms at an upper endportion and a lower end portion of the oxide semiconductor layer 701 wasfixed.

Then, the simulation result is shown in FIG. 21. In z-axis coordinates,the range of 0 nm to 1.15 nm indicates the layer 703 having low oxygendensity, and the range of 1.15 nm to 2.3 nm indicates the layer 705having high oxygen density. The distribution of oxygen densities beforethe MD simulation is indicated by a solid line 707, and the distributionof oxygen densities after the MD simulation is indicated by a dashedline 709.

The solid line 707 shows that the oxide semiconductor layer 701 hashigher oxygen densities in a region raging from the interface betweenthe layer 703 having low oxygen density and the layer 705 having highoxygen density to the layer 705 having high oxygen density. On the otherhand, in the dashed line 709, it is found that the oxygen densities areuniform in the layer 703 having low oxygen density and the layer 705having high oxygen density.

As described above, when there is non-uniformity in the density ofoxygen densities as in the stack of the layer 703 having low oxygendensity and the layer 705 having high oxygen density, it is found thatthe oxygen diffuses from where an oxygen density is higher to where anoxygen density is lower by heat treatment and thus the oxygen densitiesbecome uniform.

That is, as described in Embodiment 1, since the oxygen density at theinterface between the first oxide semiconductor layer 432 and the oxideinsulating film 407 is increased by formation of the oxide insulatingfilm 407 over the first oxide semiconductor layer 432, the oxygendiffuses to the first oxide semiconductor layer 432 where the oxygendensity is low and thus the first oxide semiconductor layer 432 hashigher resistance. As described above, reliability of a thin filmtransistor can be improved.

The present application is based on Japanese Patent Application serialNo. 2009-156414 filed with Japan Patent Office on Jun. 30, 2009, theentire contents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

-   -   34: oxide semiconductor layer, 100: substrate, 101: gate        electrode layer, 102: gate insulating layer, 103: semiconductor        layer, 107: protective insulating layer, 108: capacitor wiring,        110: pixel electrode layer, 121: terminal, 122: terminal, 125:        contact hole, 126: contact hole, 127: contact hole, 128:        transparent conductive film, 129: transparent conductive film,        131: oxide semiconductor film, 132: conductive film, 133: oxide        semiconductor film, 134: oxide semiconductor layer, 135: oxide        semiconductor layer, 136: oxide semiconductor film, 137: oxide        semiconductor film, 138: oxide semiconductor layer, 150:        terminal, 151: terminal, 152: gate insulating layer, 153:        connection electrode layer, 154: protective insulating film,        155: transparent conductive film, 156: electrode layer, 170:        thin film transistor, 400: substrate, 401: gate electrode layer,        402: gate insulating layer, 403: semiconductor layer, 407: oxide        insulating film, 408: conductive layer, 409: conductive layer,        410: insulating layer, 411: pixel electrode layer, 430: oxide        semiconductor film, 431: oxide semiconductor layer, 432: oxide        semiconductor layer, 433: oxide semiconductor film, 434: oxide        semiconductor film, 435: oxide semiconductor film, 436: oxide        semiconductor layer, 470: thin film transistor, 471: thin film        transistor, 472: thin film transistor, 580: substrate, 581: thin        film transistor, 583: insulating film, 585: insulating layer,        587: electrode layer, 588: electrode layer, 589: spherical        particle, 594: cavity, 595: filler, 596: substrate, 601:        electric furnace, 602: chamber, 603: heater, 604: substrate,        605: susceptor, 606: gas supply unit, 607: evacuation unit, 703:        layer having low oxygen density, 705: layer having high oxygen        density, 707: solid line, 709: dashed line, 104 a: source or        drain region, 104 b: source or drain region, 105 a: source or        drain electrode layer, 105 b: source or drain electrode layer,        2600: TFT substrate, 2601: counter substrate, 2602: sealant,        2603: pixel portion, 2604: display element, 605: coloring layer,        2606: polarizing plate, 2607: polarizing plate, 2608: wiring        circuit portion, 2609: flexible wiring board, 2610: cold cathode        tube, 2611: reflective plate, 2612: circuit board, 2613:        diffusion plate, 2700: electronic book reader, 2701: housing,        2703: housing, 2705: display portion, 2707: display portion,        2711: hinge, 2721: power switch, 2723: operation key, 2725:        speaker, 4001: substrate, 4002: pixel portion, 4003: signal        driver circuit, 4004: scan line driver circuit, 4005: sealant,        4006: substrate, 4008: liquid crystal layer, 4010: thin film        transistor, 4011: thin film transistor, 4013: liquid crystal        element, 4015: connection terminal electrode, 4016: terminal        electrode, 4018: FPC, 4019: anisotropic conductive film, 4020:        insulating layer, 4021: insulating layer, 4030: pixel electrode        layer, 4031: counter electrode layer, 4032: insulating layer,        404 a: source or drain region, 404 b: source or drain region,        405 a: source or drain electrode layer, 405 b: source or drain        electrode layer, 4501: substrate, 4502: pixel portion, 4505:        sealant, 4506: substrate, 4507: filler, 4509: thin film        transistor, 4510: thin film transistor, 4511: light-emitting        element, 4512: electroluminescent layer, 4513: second electrode        layer, 4515: connection terminal electrode, 4516: terminal        electrode, 4517: first electrode layer, 4519: anisotropic        conductive film, 4520: partition wall, 5300: substrate, 5301:        pixel portion, 5302: scan line driver circuit, 5303: signal line        driver circuit, 5400: substrate, 5401: pixel portion, 5402: scan        line driver circuit, 5403: signal line driver circuit, 5404:        scan line driver circuit, 5501: wiring, 5502: wiring, 5503:        wiring, 5504: wiring, 5505: wiring, 5506: wiring, 5543: node,        5544: node, 5571: thin film transistor, 5572: thin film        transistor, 5573: thin film transistor, 5574: thin film        transistor, 5575: thin film transistor, 5576: thin film        transistor, 5577: thin film transistor, 5578: thin film        transistor, 5601: driver IC, 5602: switch group, 5611: wiring,        5612: wiring, 5613: wiring, 5621: wiring, 5701: flip-flop, 5711:        wiring, 5712: wiring, 5713: wiring, 5714: wiring, 5715: wiring,        5716: wiring, 5717: wiring, 5721: signal, 5821: signal, 590 a:        black region, 590 b: white region, 611 a: gas supply source, 611        b: a gas supply source, 612 a: pressure adjusting valve, 612 b:        pressure adjusting valve, 613 a: refiner, 613 b: refiner, 614 a:        mass flow controller, 614 b: mass flow controller, 615 a: stop        valve, 615 b: stop valve, 6400: pixel, 6401: switching        transistor, 6402: driving transistor, 6403: capacitor element,        6404: light-emitting element, 6405: signal line, 6406: scan        line, 6407: power supply line, 6408: common electrode, 7001:        TFT, 7002: light-emitting element, 7003: cathode, 7004:        light-emitting layer, 7005: anode, 7011: driving TFT, 7012:        light-emitting element, 7013: cathode, 7014: light-emitting        layer, 7015: anode, 7016: blocking film, 7017: conductive film,        7021: driving TFT, 7022: light-emitting element, 7023: cathode,        7024: light-emitting layer, 7025: anode, 7027: conductive film,        9201: display portion, 9202: displayed button, 9203: operation        switch, 9204: band portion, 9205: adjusting portion, 9206:        camera portion, 9207: speaker, 9208: microphone, 9301: top        housing, 9302: bottom housing, 9303: display portion, 9304: key        board, 9305: external connection port, 9306: pointing device,        9307: display portion, 9600: television set, 9601: housing,        9603: display portion, 9605: stand, 9607: display portion, 9609:        operation key, 9610: remote controller, 9700: digital photo        frame, 9701: housing, 9703: display portion, 9881: housing,        9882: display portion, 9883: display portion, 9884: speaker        portion, 9885: operation key, 9886: recording medium insertion        portion, 9887: connection terminal, 9888: sensor, 9889:        microphone, 9890: LED lamp, 9891: housing, 9893: connection        portion, 9900: slot machine, 9901: housing, 9903: display        portion, 4503 a: signal line driver circuit, 4503 b: signal line        driver circuit, 4504 a: scan line driver circuit, 4504 b: scan        line driver circuit, 4518 a: FPC, 4518 b: FPC, 5603 a: thin film        transistor, 5603 b: thin film transistor, 5603 c: thin film        transistor, 5703 a: timing, 5703 b: timing, 5703 c: timing, 5803        a: timing, 5803 b: timing, 5803 c: timing.

The invention claimed is:
 1. A method for manufacturing a semiconductor device comprising the steps of: forming a first oxide semiconductor layer; forming a second oxide semiconductor layer over the first oxide semiconductor layer; performing a heat treatment on the first oxide semiconductor layer and the second oxide semiconductor layer to be dehydrated or dehydrogenated, whereby each of the first oxide semiconductor layer and the second oxide semiconductor layer has a carrier density of 1×10¹⁸/cm³ or more; etching a part of the second oxide semiconductor layer so as to expose a part of the first oxide semiconductor layer; and forming an oxide insulating layer over and in contact with the part of the first oxide semiconductor layer, whereby the part of the first oxide semiconductor layer has a carrier density of less than 1×10¹⁸/cm³.
 2. The method for manufacturing a semiconductor device according to claim 1, wherein the heat treatment is performed under an inert gas atmosphere.
 3. The method for manufacturing a semiconductor device according to claim 2, wherein the inert gas atmosphere comprises nitrogen.
 4. The method for manufacturing a semiconductor device according to claim 1, wherein the heat treatment is performed at 400° C. or higher.
 5. The method for manufacturing a semiconductor device according to claim 1, wherein the first oxide semiconductor layer comprises indium, gallium, and zinc, and wherein the second oxide semiconductor layer comprises indium and zinc.
 6. The method for manufacturing a semiconductor device according to claim 1, wherein the first oxide semiconductor layer comprises indium, tin, and zinc, and wherein the second oxide semiconductor layer comprises indium and zinc.
 7. A method for manufacturing a semiconductor device comprising the steps of: forming a first oxide semiconductor layer; forming a second oxide semiconductor layer over the first oxide semiconductor layer; performing a first heat treatment on the first oxide semiconductor layer and the second oxide semiconductor layer to be dehydrated or dehydrogenated, whereby each of the first oxide semiconductor layer and the second oxide semiconductor layer has a carrier density of 1×10¹⁸/cm³ or more; forming a conductive layer over the second oxide semiconductor layer; etching a part of the second oxide semiconductor layer so as to expose a part of the first oxide semiconductor layer, wherein the part of the second oxide semiconductor layer does not overlap the conductive layer; and forming an oxide insulating layer over and in contact with the part of the first oxide semiconductor layer, whereby the part of the first oxide semiconductor layer has a carrier density of less than 1×10¹⁸/cm³.
 8. The method for manufacturing a semiconductor device according to claim 7, wherein the first heat treatment is performed under an inert gas atmosphere.
 9. The method for manufacturing a semiconductor device according to claim 8, wherein the inert gas atmosphere comprises nitrogen.
 10. The method for manufacturing a semiconductor device according to claim 7, wherein the first heat treatment is performed at 400° C. or higher.
 11. The method for manufacturing a semiconductor device according to claim 7, wherein the first oxide semiconductor layer comprises indium, gallium, and zinc, and wherein the second oxide semiconductor layer comprises indium and zinc.
 12. The method for manufacturing a semiconductor device according to claim 7, wherein the first oxide semiconductor layer comprises indium, tin, and zinc, and wherein the second oxide semiconductor layer comprises indium and zinc.
 13. The method for manufacturing a semiconductor device according to claim 7, wherein the conductive layer comprises copper.
 14. The method for manufacturing a semiconductor device according to claim 7, further comprising a step of performing a second heat treatment on the oxide insulating layer. 